alt Hacker NewsISC shut down the DHCP project in 2022 (and afaik, nobody has taken it up as a fork), so it's less of a OPNSense decision and more of an ISC decision. Nothing is stopping anyone from continuing to user ISC dhcp for a long time, but people are reluctant.
OpenBSD’s dhcpd(8) is apparently basic on the one from the ISC: https://man.openbsd.org/dhcpd
Not sure this counts as a fork or when it was “reworked” by OpenBSD, though.