The readme, which contained this information prior to 12, points to https://docs.progress.com/category/whats_new.
All i can find there are some articles about new features and release notes which only contain some information about things that don't work or have changed.
I cannot find a list of fixed issues anywhere.
I have uploaded four files to the documents section of this community; Release notes for 12.0, for 12.1 and Issues addressed in 12.0 and 12.1. The files and pointers do in fact exist on docs.progress.com but we currently have an issue with the view more functionality. This should be resolved soon. Sorry for the inconvenience.