It is possible to use newer C# syntax and libraries when running on the .NET Framework 4.0 runtime, although that is up to C# 7.0 and not anything newer. It is possible to compile and run MCGalaxy for .NET 8 and it will function mostly the same, albeit you have to manually set it up so that the compilation commands work, and even then a plugin or command might not properly reference .NET 8 assemblies and thus have to add //dotnetref Library.dll to the top of the plugin file along with the normal //reference LibraryFoo.dll lines you would use with .NET Framework.
Also MCGalaxy 1.9.5.0 is out, it's just that Unk hasn't gotten to making a GitHub release.