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.