Capitalization alone would be strange because most phone autocorrects will do it automatically. That's usually how I can tell when someone is IMing on a computer or a phone. But punctuation could be weird because it's not added automatically.

On my company's internal wiki, I maintain the "dictionary" which includes a lot of our private jokes, and there's a definition for period because a coworker said she felt weird about people finishing their sentences with a period on Slack, as it felt like they were annoyed with her or something. So of course we make sure to use punctuation with her now :3

I guess the state of the modern web just means anything is a valid reason to doubt of someone's humanity anyway…