This userscript, written for ViolentMonkey, reverts the question and answer upvotes on Stack Overflow so that they are almost identical to their previous design.

New Stack Overflow vote buttons:

This user script changes them to:

// ==UserScript==
// @name         Original Stack Overflow Vote Buttons
// @homepage     https://github.com/Tyler-H/SO-UserScripts/blob/master/OriginalVoteButtons.js
// @version      1.0
// @description  Reverts post vote buttons on Stack Overflow to the previous implementation, pre-2022-06-24
// @author       TylerH
// @match        https://stackoverflow.com/*
// ==/UserScript==

(function() {
    let allbtns = document.querySelectorAll("button[class^='js-vote-']");
    let downsvg = document.querySelectorAll('.js-vote-down-btn .iconArrowDown');
    let upsvg = document.querySelectorAll('.js-vote-up-btn .iconArrowUp');
    let downpath = document.querySelectorAll('button.js-vote-down-btn svg.iconArrowDown path');
    let uppath = document.querySelectorAll('button.js-vote-up-btn svg.iconArrowUp path');
    
    // set attribute values for the SVG elements
    downsvg.forEach ( x => x.setAttribute('height','36'));
    downsvg.forEach ( x => x.setAttribute('width','36'));
    downsvg.forEach ( x => x.setAttribute('viewBox','0 0 36 36'));
    
    upsvg.forEach ( x => x.setAttribute('height','36'));
    upsvg.forEach ( x => x.setAttribute('width','36'));
    upsvg.forEach ( x => x.setAttribute('viewBox','0 0 36 36'));
    
    // set SVG path values to resize the SVGs themselves
    downpath.forEach ( x => x.setAttribute('d','M2 11h32L18 27 2 11Z'));
    uppath.forEach ( x => x.setAttribute('d','M2 25h32L18 9 2 25Z'));
    
    //remove border & revert padding
    allbtns.forEach ( x => x.setAttribute('style','border: 1px solid transparent !important; padding: 1px 4px'));
    
    //revert hover styles
    document.head.insertAdjacentHTML("beforeend",`<style>button[class^="js-vote-"].s-btn:hover { color: var(--blue-500) !important; background: none !important; }</style>`);
    allbtns.forEach ( x => x.style.setProperty('background-color','transparent','important'));
    
    //revert :focus styles
    document.head.insertAdjacentHTML("beforeend",`<style>button[class^="js-vote-"].s-btn:focus { box-shadow: none; }</style>`);
    
    //revert :active styles
    document.head.insertAdjacentHTML("beforeend",`<style>button[class^="js-vote-"].s-btn:active { background: none }</style>`);
    
    //de-emphasize color brightness of buttons while leaving brightness of score
    //allbtns.forEach ( x => x.style.setProperty('color','rgb(105, 111, 117)',''));
    document.head.insertAdjacentHTML("beforeend",`<style>button[class^="js-vote-"].fc-black-700 { color: rgb(186, 191, 196) !important; }</style>`);
})();

This is an inefficient implementation, I am sure; I rarely write JavaScript these days so there is undoubtedly room for improvement.

This userscript, written for ViolentMonkey, reverts the question and answer upvotes on all Stack Exchange sites so that they are almost identical to their previous design.

New vote buttons (shown for Stack Overflow in dark mode): 

This user script changes them to (shown for Stack Overflow in dark mode): 

// ==UserScript==
// @name         Original Stack Overflow Vote Buttons
// @homepage     https://github.com/Tyler-H/SO-UserScripts/blob/master/OriginalVoteButtons.js
// @version      1.1
// @description  Reverts post vote buttons on Stack Exchange to the previous implementation, pre-2022-06-24
// @author       TylerH
// @match        https://*.stackoverflow.com/*
// @match        https://*.stackexchange.com/*
// @match        https://*.superuser.com/*
// @match        https://stackapps.com/*
// @match        https://*.serverfault.com/*
// @match        https://*.askubuntu.com/*
// @match        https://*.mathoverflow.net/*
// ==/UserScript==

(function() {
    let allbtns = document.querySelectorAll("button[class^='js-vote-']");
    let downsvg = document.querySelectorAll('.js-vote-down-btn .iconArrowDown');
    let upsvg = document.querySelectorAll('.js-vote-up-btn .iconArrowUp');
    let downpath = document.querySelectorAll('button.js-vote-down-btn svg.iconArrowDown path');
    let uppath = document.querySelectorAll('button.js-vote-up-btn svg.iconArrowUp path');
    
    // set attribute values for the SVG elements
    downsvg.forEach ( x => x.setAttribute('height','36'));
    downsvg.forEach ( x => x.setAttribute('width','36'));
    downsvg.forEach ( x => x.setAttribute('viewBox','0 0 36 36'));
    
    upsvg.forEach ( x => x.setAttribute('height','36'));
    upsvg.forEach ( x => x.setAttribute('width','36'));
    upsvg.forEach ( x => x.setAttribute('viewBox','0 0 36 36'));
    
    // set SVG path values to resize the SVGs themselves
    downpath.forEach ( x => x.setAttribute('d','M2 11h32L18 27 2 11Z'));
    uppath.forEach ( x => x.setAttribute('d','M2 25h32L18 9 2 25Z'));
    
    //remove border & revert padding
    allbtns.forEach ( x => x.setAttribute('style','border: 1px solid transparent !important; padding: 1px 4px'));
    
    //revert hover styles
    document.head.insertAdjacentHTML("beforeend",`<style>button[class^="js-vote-"].s-btn:hover { color: var(--blue-500) !important; background: none !important; }</style>`);
    allbtns.forEach ( x => x.style.setProperty('background-color','transparent','important'));
    
    //revert :focus styles
    document.head.insertAdjacentHTML("beforeend",`<style>button[class^="js-vote-"].s-btn:focus { box-shadow: none; }</style>`);
    
    //revert :active styles
    document.head.insertAdjacentHTML("beforeend",`<style>button[class^="js-vote-"].s-btn:active { background: none }</style>`);
    
    //de-emphasize color brightness of buttons while leaving brightness of score
    //allbtns.forEach ( x => x.style.setProperty('color','rgb(105, 111, 117)',''));
    document.head.insertAdjacentHTML("beforeend",`<style>button[class^="js-vote-"].fc-black-700 { color: rgb(186, 191, 196) !important; }</style>`);
})();

This may be an inefficient implementation; there is undoubtedly room for improvement.

Note that the color may need to be updated per-site. I have it set for Stack Overflow, but I know other sites use different color schemes.

This script does not modify the styles for the post score, since that is not technically included in the up/down vote buttons, but nevertheless, users of the script may appreciate styles to revert the score to a prior design. The CSS styles I use for that are as follows:

div.js-vote-count {
    font-family: "Segoe UI Adjusted","Segoe UI","Liberation Sans",sans-serif !important;
    font-size: 21px !important;
    font-weight: 400 !important;
    color: rgb(186,191,196) !important;
}

// ==UserScript==
// @name         Original Stack Overflow Vote Buttons
// @homepage     https://github.com/Tyler-H/SO-UserScripts/blob/master/OriginalVoteButtons.js
// @version      1.0
// @description  Reverts post vote buttons on Stack Overflow to the previous implementation, pre-2022-06-24
// @author       TylerH
// @match        https://stackoverflow.com/*
// ==/UserScript==

(function() {
    let allbtns = document.querySelectorAll("button[class^='js-vote-']");
    let downsvg = document.querySelectorAll('.js-vote-down-btn .iconArrowDown');
    let upsvg = document.querySelectorAll('.js-vote-up-btn .iconArrowUp');
    let downpath = document.querySelectorAll('button.js-vote-down-btn svg.iconArrowDown path');
    let uppath = document.querySelectorAll('button.js-vote-up-btn svg.iconArrowUp path');
    
    // set attribute values for the SVG elements
    downsvg.forEach ( x => x.setAttribute('height','36'));
    downsvg.forEach ( x => x.setAttribute('width','36'));
    downsvg.forEach ( x => x.setAttribute('viewBox','0 0 36 36'));
    
    upsvg.forEach ( x => x.setAttribute('height','36'));
    upsvg.forEach ( x => x.setAttribute('width','36'));
    upsvg.forEach ( x => x.setAttribute('viewBox','0 0 36 36'));
    
    // set SVG path values to resize the SVGs themselves
    downpath.forEach ( x => x.setAttribute('d','M2 11h32L18 27 2 11Z'));
    uppath.forEach ( x => x.setAttribute('d','M2 25h32L18 9 2 25Z'));
    
    //remove border & revert padding
    allbtns.forEach ( x => x.setAttribute('style','border: 1px solid transparent !important; padding: 1px 4px'));
    
    //revert hover styles
    document.head.insertAdjacentHTML("beforeend",`<style>button[class^="js-vote-"].s-btn:hover { color: var(--blue-500) !important; background: none !important; }</style>`);
    allbtns.forEach ( x => x.style.setProperty('background-color','transparent','important'));
    
    //revert :focus styles
    document.head.insertAdjacentHTML("beforeend",`<style>button[class^="js-vote-"].s-btn:focus { box-shadow: none; }</style>`);
    
    //revert :active styles
    document.head.insertAdjacentHTML("beforeend",`<style>button[class^="js-vote-"].s-btn:active { background: none }</style>`);
    
    //de-emphasize color brightness of buttons while leaving brightness of score
    //allbtns.forEach ( x => x.style.setProperty('color','rgb(105, 111, 117)',''));
    document.head.insertAdjacentHTML("beforeend",`<style>button[class^="js-vote-"].fc-black-700 { color: rgb(105, 111, 117) !important; }</style>`);
})();

// ==UserScript==
// @name         Original Stack Overflow Vote Buttons
// @homepage     https://github.com/Tyler-H/SO-UserScripts/blob/master/OriginalVoteButtons.js
// @version      1.0
// @description  Reverts post vote buttons on Stack Overflow to the previous implementation, pre-2022-06-24
// @author       TylerH
// @match        https://stackoverflow.com/*
// ==/UserScript==

(function() {
    let allbtns = document.querySelectorAll("button[class^='js-vote-']");
    let downsvg = document.querySelectorAll('.js-vote-down-btn .iconArrowDown');
    let upsvg = document.querySelectorAll('.js-vote-up-btn .iconArrowUp');
    let downpath = document.querySelectorAll('button.js-vote-down-btn svg.iconArrowDown path');
    let uppath = document.querySelectorAll('button.js-vote-up-btn svg.iconArrowUp path');
    
    // set attribute values for the SVG elements
    downsvg.forEach ( x => x.setAttribute('height','36'));
    downsvg.forEach ( x => x.setAttribute('width','36'));
    downsvg.forEach ( x => x.setAttribute('viewBox','0 0 36 36'));
    
    upsvg.forEach ( x => x.setAttribute('height','36'));
    upsvg.forEach ( x => x.setAttribute('width','36'));
    upsvg.forEach ( x => x.setAttribute('viewBox','0 0 36 36'));
    
    // set SVG path values to resize the SVGs themselves
    downpath.forEach ( x => x.setAttribute('d','M2 11h32L18 27 2 11Z'));
    uppath.forEach ( x => x.setAttribute('d','M2 25h32L18 9 2 25Z'));
    
    //remove border & revert padding
    allbtns.forEach ( x => x.setAttribute('style','border: 1px solid transparent !important; padding: 1px 4px'));
    
    //revert hover styles
    document.head.insertAdjacentHTML("beforeend",`<style>button[class^="js-vote-"].s-btn:hover { color: var(--blue-500) !important; background: none !important; }</style>`);
    allbtns.forEach ( x => x.style.setProperty('background-color','transparent','important'));
    
    //revert :focus styles
    document.head.insertAdjacentHTML("beforeend",`<style>button[class^="js-vote-"].s-btn:focus { box-shadow: none; }</style>`);
    
    //revert :active styles
    document.head.insertAdjacentHTML("beforeend",`<style>button[class^="js-vote-"].s-btn:active { background: none }</style>`);
    
    //de-emphasize color brightness of buttons while leaving brightness of score
    //allbtns.forEach ( x => x.style.setProperty('color','rgb(105, 111, 117)',''));
    document.head.insertAdjacentHTML("beforeend",`<style>button[class^="js-vote-"].fc-black-700 { color: rgb(186, 191, 196) !important; }</style>`);
})();