The comparison with Phelps' feet is incredibly tiresome, for reasons not limited to the following:
(a) The 1980s-born male swimmer who was actually famous for having outsized feet [size 17] was Ian Thorpe, with Phelps' being 'only' size 14;
(b) Thorpe's feet (news.bbc.co.uk/sport1/hi/other_sports/831211.stm) are actually are reminder of the extent to which being a large person, with large hands and feet, confers an advantage in swimming - Thorpe, an athletic, very but not freakishly tall man, has bigger feet than the single woman on the entire planet of 8 billion or whatever it is, does (www.guinnessworldrecords.com/news/book/2019/4/woman-with-worlds-biggest-female-feet-gets-her-custom-made-size-15-wedding-shoes-568744), with the woman in question looking in photos [sorry] rather awkward and unathletic.
(c) Having separate sporting categories for foot size would obviously be a nonsense, with far too many questions over measurement [e.g. whether you measure width/length/volume, whether you factor in toe-webbedness etc]; where to draw the line [e.g. above sized 13 or similar?]; and how many entrants there'd be in each category in a given year.
(d) But if there actually was a sporting category for people with outsized feet then the likes of Ian Thorpe absolutely would have to go in it, it'd be ludicrous for him to be able to identify his way into a category for swimmers with little trotter-like feet.
(e) In those few sports where physical size has been deemed to confer an intolerable advantage on bigger people but where there's a will to get smaller people involved [primarily the combat sports, also weightlifting] & necessitating multiple within-sex categories, the weigh-in is a huge deal/can attract many TV viewers/is carried out with great precision etc. Nobody thinks that a boxer using last minute various dehydration techniques including sauna/laxatives/etc etc etc is 'cheating', [s]he's just doing what's sensible to get into an competitively advantageous category. Self identification into weight classes would be beyond nonsensical.