C# is a useful object oriented programming language. You can generally do the same stuff as you can in C++ but as a game modder there is 1 huge advantage C# has over C++ and that's the way it handles include path orders, or it's lack thereof.
I actually typed out and described a scenario where this starts messing things up but it was quite verbose. But the tl;dr is that it's possible to get stuck in a situation where you have circular include dependencies (kind of like how circular dependencies screw you over really hard in Linux package managers sometimes). If you planned the structure of your code really really well this shouldn't be too big of a problem but if you're extending something that is both complicated and wasn't meant to be extended upon, it starts becoming a problem.
C# doesn't really have this problem because instead of including header files, it does that "using blahblahblah;" business which doesn't run into include order problems.
C# is "open source" but it was invented by Microsoft and is hard to use without dealing with Microsoft. I don't want to contribute to the agenda of proprietary software in any capacity so I make all my projects in C++. C++ is very powerful but for certain gaming-oriented use cases, while it is the best choice most of the time it's not the best choice all the time.
How do I use C# in a responsible and open-source way? Do I just have to avoid using visual studio? I don't own a single Windows or Mac computer that actually boots up. Do I avoid dotnet framework? Do I have to avoid everything dotnet? What about Net Core?
Typing g## into a terminal window isn't a thing so what's the FOSS way to use C#?
I'm not particularly familiar with the C# ecosystem, only with the rather similar JVM ecosystem, and well, in that, there is a way to compile to machine code ahead of time (called "GraalVM"; the equivalent in C# might be ".NET Native").
But as I understand, that still needs a garbage collector and still does many unoptimal things, like constantly cloning strings or doing array boundary checks on every single array access.
So, yeah, I wouldn't either bet money on being able to run on an old Athlon with that.
Someone here has already recommended Rust and I'll extend that recommendation. You get that same performance/target flexibility. It's more abstracted than C++ and in particular much harder to shoot yourself in the foot with. And it's quite popular in the open-source community.
It does not use a garbage collector, so some patterns are more difficult than in C#. But since you're doing gamedev, you'll probably be using a game engine with Entity-Component-System architecture (personally, I'm using Bevy) and that bypasses a lot of those memory management gotchas.
One more downside to Rust, is that it's still rather young. I do feel like Bevy is in the generation of game engines that will probably survive long-term, but it's still doing breaking API changes frequently and you don't get a fancy GUI editor like with Godot...