I was lucky enough to have never seen that subreddit, I only started using Reddit well after it was banned, but based on what I heard, it's literally a subreddit for sharing very sexualized pictures of actual children. Not adults that look young, literal minors.
So that is considered child pornography right? Which is illegal to view, possess, and critically, distribute? And there is no way Reddit staff did not know the nature of the content being hosted from their site, so how have they not been thrown in the bin for knowingly distributing CP?