the first two didn't have a model
Genestealer cult was a few units from Rogue Trader and didn't get new models after that but yeah I suppose you could count them
lost and the damned didn't really have models and can now be used in 6th edition, allying chaos to IG
Nuandhu hasn't ever had a model
chaos cult terminators are available from FW and can be used as regular Chaos Terminators with Marks
I've never heard of cybots and can't find anything on them
Iyanna is mentioned in the supplement but never had a model either
doomrider yes, finally had a model and isn't in the rules but can still be used as a chaos champion on a bike
N'kari was never a unit in the game
Beastmen are still in the WFB but if you mean IG Abhuman regiments, you could still use those as IG and people do, Victoria Miniatures make some lovely ones.
Squats, again, no new 40k models since Rogue Trader but yeah, I guess so, but there were only ever about 4 people with Squat armies
Venoms are still in the game
Necron Pariah's ok, this is the first real unit that has been taken out of the game, you could still use the models as Lychguard as they both have the same weapons.
So, there is not much there that you would have bought in the last 20 years that you couldn't use in an army today.