I'm having trouble remembering the last time I saw a BSD box during my work day, unless Juniper gear counts. No knock against BSD but seriously how many people are writing POSIX shell scripts because they need their shell scripts to work with Linux, QNX and BSD?
A few, but there are still cases where it's best to stick with the plain old Bourne shell syntax.
When I mentioned embedded systems I primarily thought of Linux/Busybox-based devices, like OpenWRT. While one surely can have bash there, usually base image doesn't contain it.
Same story about the most common Linux-based OS out there: Android. And, while it's no one manually runs shell scripts there, apps quite frequently exec() things, so shell scripting still matters there - a tiny bit. Also, shell scripting is heavily used in firmware upgrades/patches, as well as a glue for the root/unlocking hacks.
If you plan to write a redistributable component or an useful hack for such platforms, you'd best stick with a very limited subset of POSIX.