C# 1.0 is still valid syntax, and library authors can target both legacy and current day versions of .NET with netstandard2.0 target. Most libraries do just that.

Moreover, there are still some applications that get to use modern runtime-independent language features and SDK-style projects (terse Cargo.toml-like project manifests, still XML but they are very readable) while targeting both .NET Framework 4.8 and .NET 8 at the same time (and everything in-between them). Hell, I've seen projects[0] which look relatively modern but for some reason still target .NET Framework 3.5, I do not condone that, but it is doable and sometimes there are reasons for it.

[0]: https://github.com/Kevin-Robertson/Inveigh/blob/master/Invei...