Via the forced PATH appending of bin/ebuild-helpers, portage is exposing non pms helpers for ebuilds to rely on. Shouldn't be doing this obviously; only thing exposed should be PMS bits. Should create a ebuild-helpers/internal, move the non pms crap into there, and invoke it from those pathways. preplib and prepallman being the first two candidates...
These helpers die unconditionally for all EAPIs now: prepall prepalldocs prepallinfo prepallman This die unconditionally since EAPI 7: prepallstrip prepstrip These don't die, but they currently have no consumers in the gentoo repository: prepinfo prepman
AFAICS, all non-PMS helpers in bin/ebuild-helpers die unconditionally in portage-3.0.54. So I believe that this bug can be closed.