Unicode support is great - and necessary for a lot of stupid emoticons, as well. But even without emoji there’d be dumb shit like post titles in fancy fonts, or… usernames with each letter in a blue rectangle.
That kind of thing becomes an arms race for attention.
Reddit had a ton of problems, but it was a relief getting away from people spamming HTML gimmicks in titles and comments. That is a cultural issue more than a tech issue. The same stupidity is possible even just doing ALL CAPS YELLING. Emojis specifically get namedropped and rejected because of how egregiously they stand out, and the low-effort responses they innately encourage. Like if you had a second keyboard that could only type LOL, ROFL, LMAO, etc.