I have libraries of functions written in pure sh. I use those functions in both scripts and my interactive shell (which is bash). I cannot do the same with zs

I have libraries of functions written in pure sh. I use those functions in both scripts and my interactive shell (which is bash). I cannot do the same with zsh as my interactive shell if scripting is supposed to stay POSIX/bourne shell. Which I want as I often do this in systemwide scripts or modify existing scripts (all plain sh). The incompatibilities do matter.