Earlier quoted context omitted.
Microsoft's own FOSS multiplatform implementation of the .NET runtime is now much more performant and feature complete than Mono. However Mono is easier to embed into other applications and easier to port to new platforms. That is for example why it's used for the .NET/Blazor WebAssembly stuff. Microsoft still maintains their own fork of Mono for this specific use case. Mono also implements some of the legacy Windows…
Winforms and WPF are both very maintained and supported for .NET 8 on Windows, to clarify. Good cross platform desktop frameworks include Avalonia.
https://dotnet.microsoft.com/en-us/apps/maui
I was pleased to see WinForms got some updates in .NET9. I really thought they'd left it. I still use it every day when I need to spin up a new tool to do some little task that needs a GUI.