It is not at all
intuitive that
issubclass(A, (B, C))
means "Is A a subclass of B or a subclass of C?" when it could also
mean "Is A a subclass of both B and C?".
I've used issubclass() maybe 20 times in more than 20 years programming Python. I know the correct meaning.
But sure, I can imagine not knowing which of the two things you mention is the meaning. Either could be useful. How many times would you need to look it up?!
For folks who actually use issubclass() fairly often, that came be a burden. For those of us who rarely use it, consulting the docs once a year also isn't a burden.
This just doesn't warrant new syntax.