I grew up in the Internet at that time, and it's certainly not how I type. So you might want to be more specific about which sites or subcultures you think this style is representative of?

I’m certainly no authority but i tend to write the same way for casual communication, came from the 90s era BBS days. It was (and still is) common on irc nets too. Autocorrect fixes up some of it, but sometimes i just have ideas i’m trying to dump out of my head and the shift key isn’t helping that go faster. Emails at work get more attention, but bullshittin with friends on the PC? No need.

I’ll code switch depending on the venue, on HN i mostly Serious Post so my post history might demonstrate more care for the language than somewhere i consider more causal.