Click on a release to see all issues within it, which can either be Open (meaning they are in-progress for that release) or Closed (meaning they are done).All changes for recent GameMaker versions are listed on this website. It lists release notes and downloads for the Monthly, LTS and Beta versions separately.
More in-depth changes can be found on GitHub. The linked page lists all major releases for GameMaker.
The links above contain changes for GameMaker 2023.8 and above. Release notes and downloads for older versions are available here: IDE / Runtime
The linked GitHub page has two categories of releases: Open (containing in-progress and future releases) and Closed (which are already released to public).
Click on "[N] Closed" to see the released versions:
Click on a release to see all issues within it, which can either be Open (meaning they are in-progress for that release) or Closed (meaning they are done).
If a version is in its beta period, the Closed issues have likely made their way into the latest beta release, or will go in the next beta update.
The GitHub issue lists are always up-to-date, so it's a good place to check for known issues before reporting something.