I can’t help but think that Microsoft has decided to proceed in some way that will break compatibility, so they’re done with Mono now.

It’s essentially right there in the article:

Microsoft maintains a modern fork of Mono runtime in the dotnet/runtime repo and has been progressively moving workloads to that fork. That work is now complete, and we recommend that active Mono users and maintainers of Mono-based app frameworks migrate to .NET which includes work from this fork.

We’re done with it, you guys can take the scraps. By the way, ours is better and folks should move to it.