In a discussion on LinkedIn somebody opined that PF is "an evolved fork" of IPF.
My as-polite-as-possible correction to that was,
"$poster, It's not a fork.
Rather, an independent implementation that was initially tweaked so OpenBSD kept a mostly compatible configuration syntax.
ipf was removed https://marc.info/?l=openbsd-cvs&m=99118909527873&w=2 and the initial PF import happened later https://marc.info/?l=openbsd-cvs&m=99341215821296&w=2 (with lots of commits following)."