The Mono Project (mono/mono) (‘original mono’) has been an important part of the .NET ecosystem since it was launched in 2001. Microsoft became the steward of the Mono Project when it acquired Xama...
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.
Unity3d (a widely used game engine) ships with Mono, which makes modding easy for the developers to implement and the modders to use. Many hobbyist and professional programmers like to use Linux even for gaming, thanks to Steam's Proton. But until now you can only mod on Windows. With the gifted code maybe it will be possible on Linux, too.
The gamers on Windows are happy because they get more mods, and it is also good publicity for Microsoft.
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.