Don't think all might knows, he seemed pretty clueless about the one for all. Maybe all might's master never told him.
yeah that's what I'm thinking so far, maybe she passed away too soon to tell him? Or it could have just been lost/forgotten by another predecessor
Remember how One For All called her talentless? he may have said that to egg All Might on, but maybe he also said it because she never was able to tap into the other's quirks. Since she never was able to do, she couldn't/didn't teach All might (plus her untimely death). And Since All Might never saw it, he doesn't understand much about AfO outside the super strength aspect.
Oooo that makes sense
Holy shit I can't believe I never noticed what a plothole that shit was when Allmight was explaining One for All and how it basically merges quirks and passes them down... MY LITTLE BRAIN DIDNT EVEN THINK OF DEKU EVENTUALLY USING ALL OF THE QUIRKS im so shook holy fucking shiiit
How did Allmight not discover this ahh was he just flexing on everyone all this time? (or maybe he knows but hasn't told deku yet??)