>e.g. only blocking actual criminal content, which is all youtube have done before this, right?

Youtube has always blocked porn. Youtube has had a other restrictions as well, for example at first all videos had to be under 10 minutes. In response to criticism of disturbing videos of children, youtube deleted 150k videos of children in disturbing situations[1]. Read the guidelines from 2009[2], there's various things banned there that aren't illegal.

[1] https://www.buzzfeednews.com/article/blakemontgomery/youtube...

[2] https://web.archive.org/web/20091114041327/http://www.youtub...