[ALittleLight]

Holocaust denial is offensive and frequently a precursor to Nazi ideology. Antivax leads to children not getting medicated.

Flat Earth though seems pretty valuable to me. It's an intellectual exercise to argue a challenging position, and to do so you must learn the evidence you're arguing against. Until flat Earth advocates start getting into NASA or something I can't see how they're doing any harm.

In general, I tend to think that even if some ideas are toxic and should be kept from children or the mentally disturbed, there's nothing wrong with discussing ideas of any kind between people speaking peacefully and consensually.