apple store's beta testing channel
TestFlight. lol. i hate apple. google play and steam both have betas, steam's betas are only linked to published apps tho, there isn't as much critical risk with games as there can be with apps.
yeah, steam allows a lot of experimental stuff too, it's one of their draw cards.
a lot of times, early releases on steam have turned out to enable building out a simple game concept into something quite awesome over time. most of the games i play now are this - the long dark, kingdom games, valheim. all of them were early, simple games to start with that have developed because they got a platform.