It is a ranger class feature and a paladin one. In fact for colossus slayer favored enemy rangers it is amazing. Because the slayer's momentum triggers on damage so the first attack can miss, then all other missed attacks still get to add +1d6 because that damage is "extra damage" not "extra damage on a hit".
It also stacks with the ranger spell hunter's mark for another +1d6 on a miss.
So a 5th level ranger using a polearm with polearm master feat gets 4 swings all of the counting as two handed weapon attacks a round all but the first doing str mod+2d6 damage on a miss.
Don't forget to add 2d6 if that ranger is wielding a flametongue.