Frankly I don't expect much interest in a feature like that (but I could be mistaken - it has happened in the past and will continue to happen in the future).
Why not try to make an extension yourself?
At least 1 extra field is needed in the users_table: e.g. postcount_today. I think you can probably almost copy the procedure for postcount to increment that for each post (+ decrement that if the post is deleted)
Then you'll need a procedure to set that to 0 at the beginning of the day. Which means you'll have to decide on what is the beginning of the day: your timezone or the user's timezone?
Then (just going from memory) by modifying the autogroup extension slightly (or adding), you could, based on postcount_today, autoadd a user to a group with no posting permissions, effectively preventing him from posting.
And I think another autogroup rule could be made to remove a user from that group after 1 day membership (or the start of a day): see https://github.com/phpbb-extensions/aut ... uto-Groups