So I haven’t heard anything about SUperDuper! for over a month now. With no response to an email to the developer, I can only assume by the silence that it is not to be a “Big Sur” app.
Too bad 'cause it was my friend and a lifeline for a very long time.
R.I.P. SD
I don’t think that SuperDuper is necessarily dead because the developer indicated that it would take a while until the app will be ready for Big Sur.
I switched to Carbon Copy Cloner years ago. The development is more active and it feels more like being at home in the 2020s while ShirtPocket does feel a bit as if it was stuck in 2005 also when you take a look at the website or the blog.That does not mean that SuperDuper is bad but it definitely does not invite you to jump on the SuperDuper bandwagon as a new customer.
What I am struggling to say is that SuperDuper might not be dead yet, but it definitely does not look to be in very active development, either.
Allison Sheridan did a nice blog post on why she switched to CCC last year:
Same here. SuperDuper feels more and more stuck in the past. 
Well I made the switch to CCC and I am quite happy with it. I only with is had SD’s option to quit after a backup finishes.