It goes back before that. There were well known Usenet folks who adhered to the style. The 1970s-and-earlier Arpanet was before my time, but I'm sure it existed then too ;).
You know, I was trying to remember if anyone from Usenet did similarly, but I couldn't think of anyone.
I was a bit post-Great Renaming into well post-Eternal September. And we may have followed different groups.
The style arises spontaneously in isolated individuals and groups of course (at least since e. e. cummings!), but it was pervasive-to-universal on IRC and MUDs.
I do wonder how it trickled into there though. The most boring answer is probably the correct one. it was slightly easier to type and kids are naturally flexible.
It goes back before that. There were well known Usenet folks who adhered to the style. The 1970s-and-earlier Arpanet was before my time, but I'm sure it existed then too ;).