Well, let's look at our own time. Are you extensively knowledgable of Polish dress, customs, slang, etc. from 1900? Or Greek? Or Russian? If the last 75-100 years of knowledge has been of bumpy-headed Klingons (especially since famous Klingons like Kor and Koloth have ridges again in the 24th Century) and the Klingons themselves don't like to talk about it (and probably actively suppressed images, etc., from that accursed time, once genetics overcame the effects of the virus), I could easily see people forgetting that the Klingons, for some 120 years or so, had no ridges. It's likely a medical footnote, more than anything else.
Davy Jones
"Frightened? My dear, you are looking at a man who has laughed in the face of death, sneered at doom, and chuckled at catastrophe! I was petrified."
-- The Wizard of Oz