It did come with bash, which is GPL. (sure, zsh is now the default, but bash is there too)

macOS actually comes with Bash 3.2, which is from 2007. it is the last version released before they switched to GPL. this is actually why they switched to zsh.

Well, the fact is, the version available on opensource.apple.com in macOS 11.0.1 is 123.40.1 and the version in 10.15.6 is 118.40.2... and they slightly differ.

Interestingly, the new version removed the dependency on the unavailable rootless.h header, making it (probably) buildable out of the box, now.

I would assume the version difference is from Apple updating it internally. They don't run into GPL issues unless they pull from newer upstream Bash code.

^ We all assume that's why, it should perhaps be said. Although I think the circumstantial evidence is pretty strong. :)

Not really - they are obligated to provide it on request, and allowed to take a reasonable time to do so. If someone requested it, and they later uploaded it here, they'd be fine.

In what way does the BSD license obligate that?

I read that comment as saying that GPL wouldn't require simultaneous release, but would simply obligate them to release on request.

But I may also be misreading it.

Yes, I was referring to the GPL, not BSD licensed software.