The computers that moved from OpenBSD to Ubuntu were our local resolving DNS servers. These don't use PF and we also wanted to switch from our previous OpenBSD setup to Bind, where we were already running Bind on Ubuntu for our DNS master servers. The gory details were written up here: https://utcc.utoronto.ca/~cks/space/blog/sysadmin/UsingBindN...
We may at some point switch our remaining OpenBSD DHCP server to Ubuntu (instead of to FreeBSD); like our DNS resolvers, it doesn't use PF, and we already operate a couple of Ubuntu DHCP servers. In general Ubuntu is our default choice for a Unix OS because we already run a lot of Ubuntu servers. But we have lots of PF firewall rules and no interest in trying to convert them to Linux firewall rules, so anything significant involving them is a natural environment for FreeBSD.
Why do you say OpenBSD stopped "supporting bind"? You mean they don't include it in the base system anymore since the switch to unbound?
I mean.. It's one pkg_add away. It's a weird constraint to give yourself if that was the problem, considering you absolutely had to install it on your replacement ubuntu servers.
The short version is that we wound up not feeling particularly enthused about OpenBSD itself. We have a much better developed framework for handling Ubuntu machines, making it simply easier to have some more Ubuntu machines instead of OpenBSD machines, and we also felt Bind on Ubuntu was likely to be better supported than a ports Bind on OpenBSD. If everything else is equal we're going to make a machine Ubuntu instead of OpenBSD.
I an not sure what role these computers that may transition to Ubuntu do, there are probably good reasons, I wish he had expanded on it.