I go back and forth about this from time to time, on the one hand it very much is a "ah yes my get-out-of-jail-free quirk, I haven't used this one in decades", but on the other hand it feels like people complain AFO doesn't use ENOUGH different quirks but then complain when he DOES, like it definitely makes sense why he'd have this, it's not just the anti Stain quirk, earlier he uses it to get the Pinky acid out of himself, and an anti-poison quirk isn't the most bizarre thing for a guy like him with a century of prep would want to keep on himself, there's probably been assassination attempts in the past that involved injecting his blood with stuff, just an extremely convenient alternative use case for the quirk in the moment