Man that ain't no damn myth, a freestyle IS off the top, it just so happens that the word has evolved to have dual meanings in hip hop, and written verses are ALSO classified as freestyles. And yeah, people have been spitting writtens as freestyles for a long while now, but back then it was much more taboo and unacceptable.
The shyt has evolved to something totally different now. You used to at least have to front like your shyt was off the top. You had nikkas writing their "freestyles" to mimic what a REAL freestyle would sound like. nikkas had to make sure they shyt wasn't TOO good to at least have nikkas considering the fact that it really was off the top, because you would really get called out for that shyt. It wouldn't be career ending (maybe in the old battle rap circuit) but it was definitely a taboo. Or nikkas would have a few written lines they would throw in the middle of authentic freestyles to create a hybrid freestyle. Now, nikkas just blatantly spit written shyt where it's no chance in hell that the shyt is off the top. So that's how the meaning has evolved, but make no mistake, a TRUE freestyle is, and forever will be, off the top of the dome.
And more of these rappers can go off the top then you think, although I do admit that it is becoming somewhat of a lost art, but only because nikkas don't practice it like back in the day. Some of my favorite freestylers are Common, Eminem (hate to say it cause I really don't like his music, but he was raw on the freestyle tip early on) Snoop Dogg, Mos Def, Redman, The whole Houston Screwed Up Clique (and Swisha House) movement from before everyone got deals. And I'm basically naming mainstream artist, I could go on naming more underground artist.