In January 2019 Microsoft hosted an ISV workshop for Dynamics 365 FnO. One topic discussed was how X++ APIs were deprecated; the consensus was more visibility and announcements.
As a result we have published this page:
It contains a link to a list with all the methods and metadata elements that have been marked as obsoleted per version (starting from 7.0). The list is produced mechanically - by collecting methods with the SysObsolete attribute, and metadata with IsObsolete=Yes. As such the list doesn't reveal anything new.
If this helps you make better engineering choices, please let us know.
If you expected or need something else to handle the deprecation better, please let us know.
We keep talking about incremental development; we don't talk enough about decremental development.