This is my experience as well. In the early/mid 2000s I mostly browsed sites with a small, dedicated group of regular users. These sites were (mostly) civil and had a "small town" feel where you kind of knew everyone or at least recognized their username. Even back then though, I went on a few sites where the userbase was big enough that you'd be mostly interacting with people who you may never see again on that site, and as a result it was quite a bit less civil than the small sites.

I think the big difference between today vs. back then with respect to toxicity, is now that the web is so centralized, most sites the average person probably visits (twitter, reddit, etc) involves them interacting with strangers who they may never interact with again, so there is less of a feeling like they need to "hold back" like you might when talking to someone you know you'll meet again.

Probably the only exception to this (for me at least) was Facebook, because back when I used it I almost exclusively interacted with people I knew, so there was still that same incentive to not be an asshole because you'd actually meet these people again later. These days a lot of people interact with strangers in the comments of pages/groups for like politics and other stuff, and I feel like that's where most of the nastiness is.