The rule hasn't changed.

Can you point me to this institution that decides on the rules of the English language? What's it's address? Where does it publish these rules?

There may be an informal convention among some people that using a period at the end of the last sentence in a text is passive aggressive, but it's far from universal and far from being a rule.

It is a natural result of reading both versions, noticing that one sounds more formal and has a sharp ending, and noticing that since you can write either one, if they're ending it sharply they must be doing so intentionally. If you use the full availability of communication options available, it inherently sends that signal, if you follow rules for the sake of following rules though, then it limits that option so doesn't send that signal.

Seems like it's just as pedantic to expect people who have habitually used correct punctuation for decades to adopt this convention without ever being told and then blaming them for not abandoning an immensely useful part of written language for no apparent reason.

You had literally decades to adjust and change, this isn't new, it's been the case since at least the early 00s when cell phones and instant messengers became a thing.