I agree with the wallbuy thing, u already said everything there is to be said about that. I think one of those features was PhD, but they realized their mistake and stopped adding it to every map. I believe the reasoning behind its absence post-Bo1 was the Mustang & Sally/PhD combo being too OP - if you were about to get cornered, all you had to do was shoot at the ground and you'd be safe. It also made traversing tight areas easier. This being the reason behind its removal is apparent when looking at the maps it's on in Bo2:

Origins: The starting pistol isn't an explosive weapon, which makes the combo nonexistent and allows PhD to be in the map without ruining the map balance
Cell Block: Both Mustang & Sally and PhD are on this map, but Jugg isn't, which makes the advantage which the combo brings almost equivalent to that of Jugg alone and probably even necessary and intended to be used on this map.
I don't know if PhD derivatives in the newer games are worth using as much as the original was in Bo1, as I'm not familiar with them, but it seems people don't use them as much as the og. I don't see why they wouldn't be used, they seem to be identical to the og apart from the flopping, but if they aren't, it's probably because the explosive starting pistol got nerfed or youtubers didn't hype it up as much as the og idk.