If you're a native English speaker, odds are that you didn't forget how to express subtleties like sarcarsm in writing, it's just that a lot of people on the internet are non-native English and a huge proportion of native speakers with internet access are only functionally literate (obv. in most cases not by choice). /s doesn't exist because you can't write sarcasm, it exists as a courtesy to people who can't read it.

As for the rest of it, I don't know. It's wierd that someone could forget how to inflect their voice when people learn to do it by watching TV. There's a whole swathe of English learners who learned from Peppa Pig that's triggered a surge in people speaking with a Received Pronunciation accent, along with the intonation style along with the show's child-like over-emphasized intonation. Brains are weird, but plastic enough that you can relearn stuff with persistence.