All the current BSD systems, and some GNU/Linux distributions. I'd guess that illumos does not have bash installed by default, too.

everything embedded - OpenWRT/LEDE uses busybox with ash - Android uses mksh? that only supports a subset of bash features. Every Debian/Ubuntu has ash as /bin/sh that only supports POSIX.

I'd argue that writing scripts for an embedded target for a regular server/workstation target are fundamentally different problems - almost anything I'd write for those platforms would be targeted for them - not for general purpose unix.

Portability is only desirable, if you need portability - otherwise it frequently adds complexity for little return benefit.