From a comment in the PR:
IMHO making polkit-pkla-compat optional is seriously risky. The
configuration can contain “if user=foo deny” entries, and silently
ignoring those configuration files can break the security of the
system.
If polkit indeed treats rules it doesn't have currently support for
as always satisfied, I would treat this is a blocker for splitting
out polkit-pkla-compat.
Zbyszek