WPF, a framework from 2006, and Avalonia, a 3rd party framework, are the only two worth considering.
I wish Avalonia UI's mobile solution had had Navigation sorted a year ago when I proposed that we needed to do something with the XF EOL announcement. I had a POC with it working well enough, but sadly it was "too much work" to migrate the rest of the app. Now with MAUI we have realised that we are probably doing just as much work anyway, and it is seriously half baked as an ecosystem.
As well with the rise of new tech/stacks that has displaced C# in a few places, the people who stay in C# tend to bristle at other OSes as that's what the people who are using the other techs use (and us vs. them type attitude).
So in the grand scheme of things, it's unsurprising that this happens.
Keep in mind that .NET has "inherited" a huge ecosystem and developer community that existed and still exists within the confines of .NET Framework. It roughly splits into two groups - the one that is moving or has moved on to .NET and left the legacy in the past and the one that harms the ecosystem by refusing to do so. Luckily, the former is in the minority compared to the latter especially as the time goes by.
It is also helped by new generations of developers who came into working with the tech after .NET Core and .NET became the mainstay or developers who moved on to .NET from other languages and ecosystems.
Our production environment where all applications are run is based on Debian (though we test on Windows Server as well) in a kubernetes cluster, running in Azure.
C# is decent language for writing line of business application logic, and these days .Net is a completely cross platform, open source framework.
I remember using a bunch of C#/Mono applications (F Spot, Banshee, etc, they were all the rage once) on my Linux desktop over 20 years ago.
Fastforward to .NET 5, 6, 7, 8.. it's just doing great x-plat life and innovation
I'm one amongst many using Net on other systems (Mac & Linux) as well as Windows. Larger enterprises which use VS on Windows often deploy the results onto Linux servers too (especially with microservices), which is hardly being hostile.
Though to address the issue of GUIs in Net-land, I've given up as nothing since WinForms (and possibly WPF) has lasted. They are 'supported' in many cases, but it always feels half-hearted or half-dead.