Nothing proven. Much documentary about it though.

There's a vine/TikTok dude that made a short about it somewhat recently in a shallow and humorous fashion.
Basically, the Jackson dad was an asshole who beat his kids. Michael being the most successful one got the worst of it, as per the documentary. Not sure if his brothers covered him and how often.
He didn't have a childhood, he had beatings, training and concerts.
His soft-spoken voice was beaten into him, only allowed to use his full voice for singing.
His Neverland was built for that which he never had and he tried to give that to other children he felt were like him.
Whether something happened is only known by the the people involved, but odds are that unless Michael himself was molested as a child, it's far more likely that he was deeply traumatized, cried himself to sleep and had recurring night terrors.

I'll repeat, only the people involved know what happened and they're not talking.