i don't think reddit is hosting underage porn, and im sure there is some subreddit somewhere that has questionable content (the same as there is copyrighted material somewhere deep on youtube) but i don't think that it is so widespread.
Minors are 100% uploading to popular porn subreddits. There might not be a specific r/jailbait any more(and there probably is), but it's definetely still happening. They just have "18" in the title now.
...just go ahead and scroll through r/gonewild, and you know what the OP meant.