I do not understand the limitations on AIS-31 compliant. Maybe someone who has more knowledge can help us better ( i would move this topic to technical discussion board for better inputs from community and less yobit spammers).
But there are limitations on randomness of flipping a coin over and over again.
1 - muscular memory - i don't know if that expression is correct, I am talking about you doing the exactly same movement 256 times is not totally random. Your movement may not be 50-50, but 30-70, as it is the same over and over again. Maybe if you asked to different 256 people flip the coin once, it would have a better randomness.
2 - coin may have not be completely balanced, which could also lead to biased results (not 50-50).
But anyway, as I said before, I think it is enough randomness for a priavate key generation. Don't know if it is better or not than AIS-31. But flipping a coin is not perfectly random imo.
It's certainly not perfectly random but it's safer than relying on so called "True Random" Number Generators that can be bugged or having design weaknesses, and are actually just trustful blackboxes. I think it's well known physical sources of entropy are safer than digital ones.
With the iancoleman page you can generate 24 words seeds by using 6 sided dices, 52 cards decks, binary sources(like coin flips), etc.
https://iancoleman.io/bip39/ (enable Show entropy details)