A post's timeline list all of the public events that have happened to that post, such as creation, votes, comments, answers, edits, etc.

Links to timeline pages are not normally shown to non-moderators, but the pages themselves are visible to all (but some info may be hidden).

The general URLs are of the form https://$site/posts/[post ID]/timeline and there are a variety of userscripts  available to add convenience links.

Reference:

• API's /questions/{ids}/timeline route
• The API does not seem to have the equivalent of an answer's timeline.

Sample timeline URL:
    https://stackapps.com/posts/8113/timeline (for an answer)

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