There is VS Code, which is not a version of VS, is free, and has this debugger.
There is also VS Community, which is a free version of VS, has this debugger, but has fairly strict licensing limitations (e.g. it can't be used in corporations).
Note that Mono, the framework, is indeed dying. But Mono, the runtime, is now part of .Net and it's an alternative to CoreCLR, used for things like Xamarin/MAUI and Blazor.
Though most of the time, you don't need to know any of this, you just use .Net and it works on Windows, Linux, Android, Apple and in the browser.
Exceptions are here to stay though. I don't see how you could retrofit something like Rust's Result on top of .Net. (Also, .Net has a pattern that's an alternative to exceptions: `bool TrySomething(out T result)`. But it's quite limited, in several ways.)
How do you differentiate between "actually abandoned and probably dangerous" and "actively maintained, but updated only very rarely, because there's nothing left to do"?
> While writing it I needed something that would be 'just data' without any additional 'functionality' attached to it, so I naturally reached for `struct`, given that it exists in the language. The .NET guys asked why it was not a `class` instead. See, somewhere deep inside they felt that not wrapping everything in a `class` is wrong on some fundamental level.
The difference between a class and a struct in C# is not the same as between a class and a struct in C++. In C#, structs can definitely be useful, but also have some gotchas, so they're mostly used only in performance-critical parts of the code.
If you visit a foreign tribe, I think you should listen to the locals, unless you are really certain your way of doing things is better in the local context.
I think there's at least one way in which F# is a second-class citizen: documentation. The documentation for anything in .Net will contain code in C#, but code in F# is quite rare.
So if you want to use F#, I think you still need to be able to read C# code.
MS recently made one tiny decision that was unfriendly to open source, it caused a huge outcry and a reversal within 48 hours on a weekend. I can't even imagine what the reaction would be in your scenario.
Besides, basically everything is open source now and you can't put that genie back into the bottle.