Multiple (not dozens, but several) people are posting in Apple Discussions (example:
https://discussions.apple.com/thread/252177156 ) about external SSD's that they used for booting typically for older Macs (5 years or older), but that the external boot failed starting with application of a recent Catalina security update. Since SuperDuper makes bootable external drives in a different way than directly installing like they did, maybe that issue is not relevant, but have you encountered this issue? My bootable clones (two) under Catalina were both created with SuperDuper and both seem to still work. Some of the posters there seem to feel that this is somehow intended behavior by Apple to discourage older computers from being retained.