So I've been looking through the special rules and there are several things in the brb that don't seem to apply to any current units. There may be others, but these are some. I'd be interested if anyone knows of examples where these rules are used. I'll look silly though if I've missed them
Salvo weapons. A psycannon seems a lot like a salvo weapon and is the only example of something like one, but it isn't actually listed as one anywhere. If it was a salvo gun then GKs in power armour wouldn't be able to assault after firing it, which would nerf GKs, who deserve it!
Vector dancers. None that I can see.
Split fire. They could have given this to Tau with target locks, but this special rule seems to fail to exist so hard that even rules similar to it now fail to exist.
Missile lock. Presumably this is intended to be for flyers, but no flyers have it. Possibly because it would make it nearly impossible to miss with a missile...
Torrent. Heavy incinerators and hellhounds aren't torrent weapons, apparently.
Zealot. No idea where this one comes from, but nobody has it. Might make an appearance in the new chaos legions book though, and/or for sisters.
Of course it's possible that future codexes could have these rules in for new units, but why fill up the brb with rules that don't currently apply to anything?
In many cases, they could have added these rules to existing units. Lots of them appear to be codifications of existing rules, but then the FAQs don't tell us to use them and we carry on using the old version. Odd.