Computers were still more niche, more complex and required more effort to use which acted as a built-in filter; if you invest all this effort into getting onto the forum to participate there's a higher chance that you legitimately want to contribute constructively.
Nowadays, every idiot out there has access to an internet browser and already has a social media account (and can use it to log into most other websites which implement social login for the sake of growth) and start spewing bullshit.
I have noticed a similar trend when it comes to online PC gaming. I used to play a shooter game (Crysis) back in ~2008 and the atmosphere on servers was always great; the chat was respectful, there were actual discussions happening in-game and I haven't seen any disrespect, rage or anger. Nowadays I play Battlefield and the chat is mostly empty, only interrupted by insults and the occasional server info message. Console gaming introduces voice chat and seems dominated by kids swearing their lungs out. I guess back in the day the cost of a machine capable to run these games acted as a filter, where as nowadays everyone has access to them, even those that shouldn't.
https://www.penny-arcade.com/comic/2004/03/19 (warning: language)
The server with the least lag and most players was always a French server, when it was the end of my night I'd start yelling "France is shit" into the mic until I got banned. I'd go to bed then play on the same server the next day because my DHCP IP address lease would renew overnight on my 2MB broadband ISP.
I don't know why I used to do that - we all thought it was hilarious though. We were actually quite a good onslaught and CTF team too.
I've not played computer games since.
Thanks for the nostalgia.
You wanted to be online? Park yourself at a desk. Maybe have a laptop and wifi.
But your pants weren't tingling every three minutes.
The immediacy, ubiquity, and temorseless presence of mobile internet shifts psychology (and participation) markedly.