IME that's been common or the norm in plenty of circles for more than 25 years (so probably longer). It became less common when people started phone-posting and when phones started auto-capitalizing by default.
it is inconsistent in language usage to write differently than to speak. we don’t speak big sounds, that’s why we don't write them either. and: doesn’t one say the same thing with one alphabet as with two alphabets? why does one merge two alphabets of completely different characters into one word or sentence and thereby make the written image inharmonic? either large or small. the large alphabet is illegible in the typesetting. therefore the small alphabet. and: if we think of the typewriter, the limitation to lower case characters means great relief and is time saving. and if we think further, it would be simplified by switching off upper case characters.
To be fair, we're on a geek website. Not many have default settings on any of their devices.
It think the above is consequence of a "I can afford to write all lowercase/do unconventional thing X". And by "afford" I think here more like "I don't have bosses, nor do I have to please anyone doing conventional things".
There was an article or a discussion here a while ago how in a organizational pyramid the people on the botton usually write as normal/nice as possible, while going upwards; people can afford themselves to write however the like, including being super rude if they choose so.
