tips on installing old Audio Unit Plug-Ins on macOS 10.15 Catalina:
Apple added more security restrictions to macOS 10.15 Catalina. unless you specifically "allow" each Audio Unit plug-in (AU plug-in) that are not code-signed and/or notarized, it won't show up in the AU host apps such as GarageBand.
if you never installed the AU plug-in that you are going to install, make sure to "allow" the new AU plug-in in the General tab in the Security & Privacy pane in the System Preferences app after installing the AU plug-in and launching the AU host app such as GarageBand.
1. install (drag & drop into) the AU plug-in to the /Library/Audio/Plug-Ins/Components/ folder
2. launch your AU host app such as GarageBand
3. macOS will ask you if you want to move the new AU plug-in to Trash, click "Cancel"
4. open the System Preferences app, click the Security & Privacy icon, click the General tab, and click "Allow"
5. try to open the new AU plug-in in the AU host app
6. macOS will ask you if you want to open the new AU plug-in, click "Open"
7. now you can use the new AU plug-in in your AU host app such as GarageBand
the sequence of the events above might be different depending on how the AU host handles the AU plug-ins.
if you missed any of the steps above and the new AU plug-in doesn't show up, trash the new AU plug-in, trash the AudioUnitCache folder in the Caches folder in the Library folder in your home folder, emptying trash, and do the steps above again.