Fun thing is the current explaination regards readonly locals: https://github.com/dotnet/csharplang/issues/188#issuecomment...

It is basically: if we introduce it, it gives us a tiny benefit for a gigantic amount of noise during every single code review. It boils down to comparing developer productivity vs. language safety.

I find the argument a bit weak since code review guidelines can end this conversation AND C# has already quite some code analysis code fixes to auto-fix language usage stuff (like file scoped namespaces or nullability checks). And that case of const and readonly can be algorithmically checked.