I think Sam Altman popularized it with his tweets during the height of OpenAI, GPT popularity ~2023. Or maybe it was already trending by then but at least for me he was the first among prominent people to be doing it.
I suspect that this is just something that happens naturally, for shorter-format messaging typing in sentence case probably offers no added readability and tends to get dropped over time.
https://news.ycombinator.com/item?id=1000015
https://news.ycombinator.com/item?id=59004
https://news.ycombinator.com/item?id=15 (from literally the first HN thread ever)
Also, it's perhaps somewhat hilarious to suggest that someone who has been chronically online for decades has to do anything to "fit in"
He writes as people who grew up on IRC and similar platforms tend to.