So was 2020 the peak of intelligent commentary on the internet? I seem to recall that the quality of discourse was much higher around the year 2000, even though average IQ was higher in 2020.

I think this is a red herring. It used to be harder to access the internet, and it selected for a certain type of person. Today, in the era of the smart phone, virtually everyone in wealthy nations is on the internet. Most notably, that includes vast numbers of children.

I bet if you looked poorer countries where few people have internet-connected devices, the quality of discourse is much higher. In particular, there are likely few children participating (and the ones who do likely have unusually proactive and involved parents).