Most controversial subs which have been banned have been so because the moderators tolerated users' discussions of explicit plans to commit violent crimes; /r/incel is a good example, or because the subreddit organized harassment campaigns, such as with /r/fatpeoplehate. Other subreddits like /r/shitredditsays and /r/the_donald complied with admins' orders to keep their garbage in the dumpster and have been allowed to thrive. Then of course there is the "do not sexualize minors" rule, which I think is quite clear. It's not perfect but there are subreddits which openly advocate eg fascism or communism so the rules do work in some sense. /r/againsthatesubreddits is unintentionally a good catalogue of the ways that the policies allow for more-or-less free expression while safeguarding the site from lawsuits. I find it all grotesquely impressive.

The big 2 that I heard about are the fappening and deepfakes.

Anyone with legal/pr experience care to give us some insight what you think reddits decision to ban those subs was based on (like were they actually illegal or just banned for bringing bad PR)?

I believe the fappening essentially amounted to copyright infringement, since Lawrence owned the rights to the pictures. Deepfakes is a little more questionable, because it's a very strange topic. However, faking a nude photo of someone could essentially be considered equivalent to claiming "this person took a nude photo which looks like this", which, being false, injurious, and appearing in written media, amounts roughly to libel, I believe. But I am not a lawyer so those are poorly-educated guesses.

Well they are not a bastion of free speech... They are a company. Create your own website if you want free speech (as far as you consider it ).

I could have been clearer, I'm aware they're a private company and under no particular obligation to the preservation of free speech principles. My point was that if we're comparing hosting your own site to operating on someone else's platform, it seems like there's marginally more freedom when hosting your own website.