> Maybe allow moderation at the sub-community level (as Reddit does with its subforums), but no centralised censorship

Reddit most definitely has centralized censorship. They have additional subreddit censorship left to the discretion of subreddit moderators.

Reddit, like most social media sites, have been slowly moving from intermediary platforms to edited platforms.

> 'Cause I think the likes of Facebook, YouTube, Twitter… would rather choose the "intermediary" path than risk taking legal responsibility for their enormous user base.

They prefer the intermediary position, but are pressured to act as editors by politicians and the news companies.