Is it possible to choose RAISERROR or THROW depending on SQL Server version?
No this isn't possible.

This is invalid syntax in earlier versions and will cause a compile error.

It is not possible to hide the THROW in an EXEC inside the catch block either as a parameterless throw must be directly contained inside the catch.

You would need to deploy the code version you want according to the version of SQL Server you are deploying to (and unfortunately there is not good support for this either in the SSDT tooling that I am aware of - no equivalent of including code lines selectively through conditional compilation)