Not really, you can make .fish scripts or call “bash script.sh” when it’s more appropriate to be posix compliant

Not really. I hate writing bash scripts anyway so I usually script whatever I need in Ruby which is far better at scripting than any posix script ever will be, and even when I do need to run a bash script I still can or just shell down into bash or zsh if I really really need to. But that’s like once a year.