https://fedoraproject.org/wiki/Changes/polkit_recommends_pkla_pkexec
See in progress PR: https://src.fedoraproject.org/rpms/polkit/pull-request/2
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