I think the older generation got used to the stereotype that if people were posting with emojis, they would naturally be making more immature posts (being younger).
That’s interesting because I would have suggested the opposite. I learned to associate emojis with older internet users (boomers and up). I always understood Reddit’s anti-emoji thing to be a kind of anti-boomer gatekeeping. It had a kind of “take your Minions memes and go back to Facebook, grandma” kind of vibe.
Reddit definitely does/did hate emoji though. I think it was even part of a written down “reddiquette” at some point.