putting to one side the higher questions of "should they" (which mostly relate to the continuing discussion on age gating) reddit has an nsfw flag which supposedly marks per-post. And, nsfw marker for the entire account. the utility of a 2-level marker is pretty clear, because some people post occasional things which might cause distress/offence, and others are only there for nsfw.

The per-post thing (for example) works on reportage of the war in Ukraine: you can avoid the gore, and focus on the discussion because the imagery which is going to include things you don't want to see is fuzzed out.

I think its a good balance between editorial policy, and user-choice.