Each release of GameMaker Studio 2 comes with it's own set of release notes. These contain detailed information on the changes that have been made as well as a complete list of all the bugs that have been fixed. This is an invaluable tool for deciding whether to update your current version, or when you find something that may be a bug after a release.
The release notes are split into two different sets due to the fact that both the IDE and the actual Runtimes can be updated independently. You can find the current release notes for the IDE from the link below:
You can find the current release notes for the Runtimes from the link below: