Legacy System Problem
How to get rid of a legacy extension that is not compatible with iMac software
You do nothing. The legacy extension "warning" is getting blown all out of proportion. See:
About legacy system extensions - Apple Support
Only developers can deal with this. As the article says, you can contact a developer to ask if they ar dealing with it, but it does not affect macOS Catalina or older.
⚠️ BIG RED FLAG!!! Currently, Apple are partnering with third-party developers to use some extensions in as system components in macOS Catalina and older. IF you start removing stuff now, you will have a non-functional computer!