Knee seems to be rare-ish but not super rare? I've had ~200 fights now and only 2 have ended by "KO (Knee)'. 1 W and 1 L
https://www.mmatycoon.com/fightcommentary.php?FTID=1127760
https://www.mmatycoon.com/fightcommentary.php?FTID=1102246
What about rare submissions? Rarest one I've seen is Americana but that might more common than I think?