the lack of capitalization (and occasional omission of punctuation) was already a big thing on tumblr / twitter 10 years ago, especially in some anime and LGBT-adjacent spaces. I don't think jyn got it from Sam Altman, and I don't think he had that big a role in popularizing it.
Before smartphones, it seems to have been the standard for just about every form of instant messaging. Well, morse has no capitalization and telex would typically have been uppercase-only too, but I think we can still count those?
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.
