Thankfully they are actively working towards upgrading, Unity 6.8 (they're currently on 6.4) is supposed to move fully towards CoreCLR, and removing Mono. We'll then finally be able to move to C# 14 (from C# 9, which came out in 2020), as well as use newer .NET functionality.
https://discussions.unity.com/t/coreclr-scripting-and-ecs-st...
One annoying piece of Unity's CoreCLR plan is there is no plan to upgrade IL2CPP (Unity's AOT compiler) to use a better garbage collector. It will continue to use Boehm GC, which is so much worse for games.
Why wouldn't they use the GC that comes with the dotnet AOT runtime?
Probably because the AOT runtime doesn't run on game consoles, straight out of the box.
Capcom has their own fork of .NET for the Playstation, for example.
I don't know what kind of GC they implemented.
They just haven't announced any plans to do so yet. They might one day.
They will not be using .NET AOT probably ever though. Unity's AOT basically supports full C# (reflection etc) while .NET opted to restrict it and lean more on generated code.
For several years now, I wonder if it will ever happen.
Well, this is at least an update from earlier this month with a clear roadmap of how they're going to get there. There's hope!