Babelson Audio [French]
To get AU plug-ins working on Catalina, developers of third-party Audio Unit plugins need to issue updates. Once the software is properly updated, the plugins will again work as expected on Catalina. You should be sure you’ve got the latest versions for all your Audio Units.
If you have a third-party Audio Unit which does not work on Catalina, you should check with the developer to see if they have an update, and to request one if they don’t.