> Twitter and all forms of instant messaging (SMS, WhatsApp, Discord, and the older ones like AIM/MSN/ICQ) have normalized it for many years.

Half true. In SMS it was just easier, but in IM it mostly was a thing because the IM client's message boundaries acted as markers for beginning/end of sentence, making the formatting unnecessary. That's why using correct capitalization and periods for single sentences came to be associated with a more formal/serious tone, it was unnecessary so including it meant you wanted to emphasize it.

Even back then we'd use regular formatting outside of IM or when sending multiple sentences in a single message.

> He's adopting the native culture rather than setting a trend.

If this was the intent, it's really coming off as that "Hello, fellow kids" meme, rather than genuine.