I can’t type the number twenty on iOS

I do actually. Interesting.

Similar to diction in verbal communication, use of an acronym or capitalization in text communication can convey different meanings. In this case it can be formal vs informal but the exact meanings here differs between social groups.

To be an armchair theorist with only anecdotes real quick, I could see this phenomena maybe being a way of adjusting to communication without body language; choosing between a period or not being a rough equivalent to relaxing with a friend or having a serious discussion. This is at least how I see it in most groups but there are enough exceptions it wouldn't be fair to merely call them exceptions.

honestly, it's the exclamation point that drives me crazy. no one has ever intentionally written "On my way!" with capitalization and exclamation point literally ever.