Nothing is stopping you from constraining the language version you want to be used in your projects:
https://learn.microsoft.com/en-us/dotnet/csharp/language-ref...
You can force it all the way down to ISO-1/2.
If this is still insufficient, then I question what your goals actually are. Other people using newer versions of C# on their projects shouldn't be a concern of yours.
Easily said, applies to complaints in other languages as well, this is only doable for people that work alone.
Oddly antagonistic take on a reasonable comment. GP could be working together with other people, for example, in which case every such idiosyncratic configuration introduces a little social and mental friction. This is covered at length in similar conversations in Go threads, where people say things like “defaults matter”.
Obviously you’re not alone to disagree, and there are even some good arguments you could potentially be making. But to say “I question what your motives really are” and tell someone what they should be concerned with is… odd?
It’s a very common position with ample practical examples. While there certainly are valid counter arguments, they are a little more involved than “nothing is stopping you.” There is. Collaborating with others, for example.