Reddit is too easily censored by biased community moderators. Instead of a flagging model, comments and threads can be completely removed, with no evidence of their having existed.
a) How do you determine which subreddits need to disclose that information?
b) How do you put something as abstract as what warrants moderation in a community into a simple description? Codes of Conduct can help with this, but as I'm sure others will inevitably point out, those can be abused by poor moderators as well.
Of course, this does work in some places - Wikipedia is extremely transparent about its moderation actions and policies - but I don't think it should be the default for every single community.
The purpose of a community is (generally speaking) to provide a place for people to engage and interact with each other, not to 'benefit' any one particular group of people. Users should have the freedom to discuss what they want, yes - within reason, as long as it doesn't jeopardize the core goal of the community. Moderators need to use their best judgement and knowledge of the community to determine what meets that criteria.
Reddit has a rule about not celebrating the death of someone, which they very selectively enforce.
They allow people to cheer the death of the likes of Adolf Hitler or Muammar_Gaddafi and some random soldier in the Ukraine-Russia war, but they're quite picky about other stuff.
I'm sure they'd clamp down more on this one if they could but the people have spoken and there are simply too many people celebrating this death.