PFO interfaces have been introduced long time ago when there was a desire to improve the compatibility between FusionForge and Tuleap.
However this was utopic since forges in the PlanetForge (hence PFO) could never match Tuleap development pace 🏎️
Furthermore FusionForge seems to not be longer alive.
There is no reason to keep those interfaces that looks more of dead code than anything useful today.