Looks like the top answer (can't comment, not enough rep :) is no longer a viable solution because /System/Library/PrivateFrameworks/ToneLibrary.framework/Versions/A/Resources/Ringtones/ is now a read-only filesystem. So even with System Integrity Protection disabled, you cannot copy the custom ringtone to that folder.

To me, this other answer is no longer a viable solution on Catalina.

Specifically, /System/Library/PrivateFrameworks/ToneLibrary.framework/Versions/A/Resources/Ringtones/ is now a read-only filesystem. So even with System Integrity Protection disabled, I can’t use these steps tocopy the custom ringtone to the indicated folder.