The FreeBSD implementation of pf is derived from OpenBSD 4.5. It was modified to be capable of running in multi-threaded kernel. The implication of it not being maintained isn't quite right.
pf in FreeBSD is still maintained, but it's not the pf from OpenBSD that everyone knows and loves.
https://svnweb.freebsd.org/base/releng/12.2/sys/netpfil/pf/p...
> hostile towards to the OpenBSD project and developers for many years.
What happened and Why ?
OpenBSD kept making snide remarks at FreeBSD for not keeping the pf codebase in sync. FreeBSD also didn't import the newest version the last time they synced because a slightly older version was faster.
Then FreeBSD hard forked pf and made it multithreaded.
Now FreeBSD's pf is the fastest implementation of pf but is missing the latest features/syntax and may have some bugs that were already fixed.
Meanwhile IPFW is still faster than pf anyway, so I don't know why people even care.
https://www.openbsd.org/faq/pf/