The last time I saw FreeBSD used in a CTF, it was FreeBSD 6, back in the Kenshoto days. FreeBSD has come a long way since then.
Nowadays, other than the odd ARM problem, or esoteric reversing challenge, everything is Linux. It doesn't matter anyway, though, since CTFs are designed with the express purpose of being insecure.
I'm as big a fan of OpenBSD as anyone, but there's no reason someone couldn't intentionally cripple OpenBSD to use as a platform for CTFs.
I've only been playing CTFs for 2 years (iCTF 2013), but I don't think I've seen FreeBSD there once. In my experience, it's 99% Linux, with some occasional Windows and Android. Then there was last ruCTFe making a point of using some more obscure systems - Minix, OpenIndiana, etc.