Now, don't get me wrong: I have grave suspicions there is currently only one actual implementation of them (I don't count hobby, or abandonware, ones) but IMHO "actual standard" combined with "for real reference implementation" is way better than just reference implementation
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since I'm still within the edit window, the MAUI referenced by the sibling comment is MIT licensed https://github.com/dotnet/maui
If we want to be really pedantic they said ".NET" and not ".NET Framework" or ".NET Core", so basically that what came with .NET 5 and newer.