This bug has actually been fixed, minutes after [it was reported on MSE](https://meta.stackexchange.com/a/338347):

> Thanks for reporting! A fix for this was just deployed, and the CORS headers are getting sent again.

A more detailed explanation is available [on an answer to the same bug report on MSO](https://meta.stackoverflow.com/a/391352).

The snippet provided by Brock Adams correctly fetches all 300 pages without gettting any CORS errors.