They added real time chat a few years ago but never made it part of the api so it was only on new reddit and the official app. I only remember it exists when I’d be on my pc on reddit the odd time and see an additional notification.

I only noticed it on rare occasion when I used new.reddit.com. People used it! I had messages there everytime whereas I almost never got DMs to inbox. Occasionally if it wasn’t super old I would respond for a little while and they were always real humans.