Some have ridges, some don't. Just never saw it in TOS because Kirk never bumped into one, didn't mean they didn't exist just that we never saw the crew meet one. We don't see every single moment of their lives.
Except in DS9 we saw the return of the TOS Klingons, Kang, Kor, and Koloth...but now they all had forehead ridges for some reason.
As if they were always that way, per Gene's explainarion...or in the 24th century, they found a cure for the Augment virus.
Discovery seems to be taking a new path from TNG, DS9, and ENT, and saying that what we saw in TOS was just an interpretation of how things looked. Where as the other 3 series showed the TOS era looking pretty much like it did in TOS.
