There are two specific cases where you do not have BASH.

1. Ubuntu/Debian /bin/sh is the Debian variant of the Almquist shell, known as dash. The only non-POSIX element it includes is the "local" keyword (afaik).

2. Busybox, with the Almquist shell. Busybox also has a "bash" which offers a few cosmetic improvements to ash (an alias of [[ to [ is one that I can see in the source). I write POSIX shell scripts on Windows with busybox quite often.

The only other shell that omits everything outside of the POSIX specification is mrsh. I don't think mrsh is widely-deployed in any major distribution.

(You also aren't going to have bash in AIX/HP-UX and maybe a Solaris base load, but as the parent article says, we aren't talking about dinosaur herders.)