That's not the approach they've taken for other cross-platform work. Doesn't sound very efficient either.

It's very efficient for Microsoft. Evaluating pull requests for a different OS requires entirely different QA. At the end of the day, it's open source. Fork it.

You cannot contrast a Windows specific piece of code like WPF with projects that were created with original cross-platform support like .Net Core, it is an apples and oranges comparison.

If they were starting WPF again today they might create an agnostic master, which has a Windows specific branch. But they aren't restarting WPF, they're just open sourcing what they already have.

The difference is that with .NET Core they are committed to providing support for the cross platform implementation. With WPF they don't want to provide any official support or the idea that they might, so they're leaving the implementation up to 3rd parties that would be responsible for keeping the whole system working with any upstream changes.

They don’t intend these to be cross platform, so what they do with cross platform work isn’t really the issue.

Suppose someone came up with .NET bindings for MacOS GUI controls, or Gnome. Would you expect a Microsoft to adopt those into .Net Core?

MicroSoft have two roles here. On the one hand they are the custodians of the cross platform .NET Core system. In the second role they are the Windows platform owner and provider of .NET bindings for some platform specific features.

If you decide to contribute to .NET Core, you join them in the first effort. If you come up with Gnome bindings or such, you are in the latter role but just because you are creating those bindings you’re under no obligation to port Gnome to Windows or anywhere else. Bindings for existing platform specific features are clearly in a separate category from shared cross platform features.

Easier to conveniently forget about some repos

At which point it's easier just to use Eto.