No they aren't. The cheating rumor started with a cryptic tweet of the World Champion at the moment, Magnus Carlsen, just right after losing against Niemann and withdrawing from the tournament. The tweet didn't say or insinuate anything about anal devices. The rumors increased when they played together a few days later and Carlsen resigned in move one, again without mentioning or insinuating anything about anal devices. And finally a couple of weeks later Carlsen explained why he thinks Niemann is cheating. You can count how many times he mentions buttplugs, anal beads or whatever. Yes, zero times.

These are the serious rumors. Serious as in if it were proven true, his career would end forever. Serious as, a large part of the chess community believe that they are true or at least contain some truth in them. Serious as in tournament organizers don't send him invitations and affect his chess career in a negative way. Serious as in that still today, some players (for example Vladimir Kramnik) refuse to play against him.

The buttplug rumor started in a twitch chat as a joke, and was popularized in the satirical subreddit Anarchy Chess as a joke (believe me, I was part of the subreddit and created a couple of memes myself). It was just making an absurd situation even more absurd, nobody actually believed that he cheated via anal stimulation.

People making memes on the internet isn't a serious rumor. Real life consequences is.

I don’t think there is a single person that has heard he cheated but doesn’t know about the butt plugg.

That doesn't mean that the buttplug thing was a serious rumor.