We currently just started to write class libraries in .NET Standard so we can use this for WPF, Winforms and .Net Core with Ubuntu Server (but then without GUI). .Net 5 does not support Winforms or WPF for our .NET 4.6.x applications.
My question is, will .NET MAUI be compatible with .NET Standard 2.0 class libraries?
When I look at the .NET Standard website its says compatibility with .NET 6.0, but I can't find if that also the .NET MAUI framework.
https://dotnet.microsoft.com/platform/dotnet-standard#versions
My question is, will .NET MAUI be compatible with .NET Standard 2.0 class libraries?
Yes, because, as it was mentioned in comments, .NET MAUI is just a library. You don't compile with MAUI, but rather with .NET 6.
All .NET Core versions starting from 2.0 can reference .NET Standard 2.0 libraries, and since .NET 6 is just a rename of what would be .NET Core 5, that is certainly possible.
The problem, however, is here:
We currently just started to write class libraries in .NET Standard so we can use this for WPF, Winforms and .Net Core with Ubuntu Server. .Net 5 does not support Winforms or WPF.
You can't use WPF nor Winforms in Linux/MacOS. You also can't use .NET MAUI in Linux as there is (currently) no target for that environment. The desktop environments supported are the same as for Xamarin: Windows and MacOS.
Notice that it was mentioned here that support for Linux is being considered for .NET 7, which should be released in around a year.