Makes sense. Mono was necessary in the "old .NET" world, where runtimes were tied to Windows versions and the framework was a pure Windows framework. Mono made it possible to run old dotNET framework versions (up to 4.8) on other OSes.

Since dotNET Core and then dotNET 5 and higher, the framework itself is cross-platform so Mono is not necessary anymore, except for backwards compatibility for apps that use a now unsupported framework.

So it makes sense that Microsoft, after dropping the old dotNET Framework versions, also wants to stop supporting the cross-platform library that was only needed for those old versions.

what do you mean modding was only possible on windows? I assume that you don't mean the skyrim approach of just downloading and unpacking, which has always worked under linux too. As do tools like mod organizer.