[neonsunset]

It depends. Some projects can be retargeted to netstandard2.0/net6(8).0 without any changes at all, some require minor update of dependencies and some other - do face breaking changes in library code because APIs did get deprecated.

With that said, neither IL nor C# itself had any changes breaking forward compatibility (aside from a very early change to foreach recently discussed here).

[pjmlp]

C# isn't used in isolation without standard library.

Also, "it depends" isn't the same as "If you feed 10 year old C# to a modern compiler it will compile it fine."