Well, I finally saw the ENT episodes that explain why there are smooth-headed, swarthy Klingons in TOS but not in other Star Trek eras. I think the explanation is plausible (w/i the realm of science fiction plausibility) and seems to make sense to me generally. Although the ep doesn't explain whether all Klingons were infected w/ this genetic agent or just some of them. Also, we still don't know how the Klingons cured themselves of this genetic alteration.
However, I still can't figure out why this knowledge was completely forgotten by the Federation in the 24th century. And why are most Starfleet personnel unaware there once existed smooth-headed Klingons in the 23rd century?
Clearly, Starfleet had encountered Klingons in the TOS era & saw what they looked like. I'm sure there are visual records of Klingons from this era in the Starfleet database. But by the 24th century, it seems most Starfleet crews didn't realize the Klingons were once swarthy & smooth-headed (as evidenced by the DS9 ep Trial & Tribblations).
So it seems there would be still some questions to be answered in any future Star Trek series.