SD, Big Sur and other drives
 

I have my music on an EHD, and I've used SD for years to backup that EHD to a different EHD. I understand that SD is not currently compatible with Big Sur, but can it still be used, under Big Sur, to backup my music as described above?

Yes, it will work fine for non-startup volumes.

In Big Sur, can I use SD to back up a startup drive, and then boot from a 3rd drive and restore from there? If not, do you have any kind of estimated timeline, or perhaps a suspicion that Big Sur may not be possible for SD?

No, SuperDuper cannot currently be used to back up a startup drive. I do not have a timeline for you; as I've said elsewhere, we're gated on factors that aren't under my direct control.