(There's zero evidence it actually was an anal device; no one knows how he might've cheated if he indeed cheated. It just became a big meme.)
I do think Niemann may have cheated in OTB chess before. But not at the Sinquefield Cup, I just don't see it. And the only evidence is insinuations from a sore loser who's well known for throwing tantrums when he doesn't get his way.
The buttplug stuff was really just a joke from reddit, no one's seriously suggested anyone is actually cheating using a buttplug. Doesn't even make sense as a way of cheating, really. It would be extremely obvious.
Specifically, move 19. Qd3! [2], appears to hang the knight. When asked how he'd respond if his opponent simply took the knight (19... dxc4), he says he is "completely winning", then proceeds to give some moves that lead to a flat-out losing position. (Eg: Any GM or sufficiently strong engine agrees that r2q1rk1/1pbn1p2/2p1b1pB/p3P3/P1p2P2/2P3QP/1P4P1/3RRBK1 b - - 0 1 is lost for White.)
To be fair, he's been playing incredibly well recently, and no other evidence of cheating came out, so I'm inclined to believe it was just a "bad interview."
[1] https://www.youtube.com/watch?v=rI9jAU0jhJU
[2] https://www.chess.com/events/2022-sinquefield-cup/04/Niemann...