I believe they are independent and therefore should be compatible. Can anyone please confirm?
The short version is: it’s not “supported”, and whether it will work depends on which features you want to use.
You can use <LangVersion>9.0</LangVersion>, and VS even includes support for suggesting a language upgrade.
However, there are three categories of features in C#:
features that are entirely part of the compiler. Those will work.
features that require BCL additions. Since you’re on the older BCL, those will need to be backported. For example, to use init; and record, you can use https://github.com/manuelroemer/IsExternalInit.
features that require runtime additions. Those cannot be added at all. For example, default interface members in C# 8, and covariant return types in C# 9.”covariant return type” needs support from the runtime.
Thank you for the detailed answer.