how do you remove a legacy system extension that is not meant to be on macOS?

You do not. No doubt you are referring to a notification you received on your desktop. By the time the next operating system is released, very likely any of the "old" non-compliant system extensions will be updated by the app vendors, so don't worry about it.

If you remove a Legacy Extension from an app on Catalina or earlier, the app will no longer run.