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...