I don't see any benefit in entertaining flat-Earthers in discussion.

If you prohibit arguing about the shape of the Earth, you're banning people from explaining that EGM08 is generally more accurate than EGM96—and where it isn't. That is a significant harm. Trolls advocating obvious nonsense like flat-Earthism isn't a significant harm, because nobody over the age of 6 will be misled.

Even if you were right that debate on the shape of the earth had no benefit, forbidding it still wouldn't be science. Science is not coextensive with beneficial things.

Fully grown adults believe the Earth is flat or that we have never been space to and dismissing them as just trolls is doing the same thing you are accusing me of doing and not allowing "space for opposing views."

There do exist paranoid schizophrenics, yes. Science generally doesn't have much trouble dealing with them, unlike, for example, institutional censorship regimes.

I am not, in fact, denying you space for your views. I'm giving you the space for your views and explaining to you why they are incorrect.

This is precisely what I was talking about when I said that reddit turns all communities into echo chambers.

If you assume that all opposing opinions come from flat-earthers and idiots that couldn't possibly be right about anything you will never even think about changing your opinion on anything. You'll continue to chat with other reddit yes-men and pat yourselves on the back about how you're all so right.

The upvote / downvote self-censorship system simply does not work for any serious discussions. It might be ok for sorting the snarkiest comment under an article but that's about it

You're contradicting yourself. You yourself say there is plenty of interesting discussion but what interesting discussion is there on reddit if it is just people patting themselves on the back?