Yes, theyre legal in MTG, including organized play
[27] [28] Each player builds a 58-card singleton deck [26] along with selecting an "Oathbreaker (a planeswalker card) and a Signature Spell (an instant or sorcery) that matches the color identity of the Oathbreaker" to go with the deck
Similarly, all opposing Planeswalkers enter with half as many Loyalty Counters, which can keep them from using their most relevant abilities right away
Additionally, its hidden ability, Guts, allows it to become a formidable opponent when inflicted with a status condition, showcasing its versatility in battle
A common card with the right ability can be worth more than some rares
The natural scarcity of the Aquapolis set, combined with its age, has made it a very desirable set in the modern age of Pokmon TCG collecting, and just like most releases from that decade, it's considered one of the holy grails in the collectibles space at the moment