Slightly older, but also slightly more fundamental to the system, you need look no farther than Solaris's `/bin/sh` for an illustrated example of the pros and cons of maintaining backwards compatibility. It was a pain by, say, 2007, to have to check if you were running on SunOS and use `/usr/xpg4/bin/sh` (IIRC) if you wanted a POSIX shell--by that point the other major vendors (and Linux) had something POSIX as the default shell. But Sun had an installed user base with a bunch of shell scripts that depended on pre-POSIX semantics.
Of course, people don't do a particularly good job of writing POSIX shell either. Just this past week one of our refactoring projects at the Rubin Observatory had some difficulty because Debian uses dash as its non-interactive POSIX shell. Some bit of some conda package that we need claimed to work with dash (and POSIX generally) but in reality didn't. But it always worked fine on bash and zsh, so no one had gotten bitten until we did.
Adam