It seems to me the basic ActivityPub specification is written from the perspective of Mastodon and Twitter-like fediverse instances.
I assume Lemmy and kbin did extend this with some more objects or at least agreed how to use the existing objects and activities there to model a link aggregator with comments on top of that.
Is there some sort of specification or design document about this somewhere? All I found when googling were some old links that resulted in a 404 and the current Lemmy documentation seems more focussed on users, admins and developers and less on the protocol side of things.
With the specific concern/idea you are talking about, I have suggested the use of a system like (or just ripping-off wholesale) the Mastodon Content Warning system in a discussion where someone was advocating a “porn” tag in addition to the NSFW one, which I thought was not going to be flexible enough.
I was mostly concerned with the fact that most types of porn and other NSFW content are probably of the type that some people will seek out and some people are indifferent to and some people absolutely don’t want to see. So some sort of simple labelling with a warning would probably not work so well.
My idea was that it would probably make the most sense to label posts with their content in some sort of machine-readable way and then allow users, communities, instances,… to express the content they want at all (or don’t want) via some sort of system of filter rules. This would also have the advantage that the same tags could be used to search for content people do want. Maybe communities or instances could even express requirements for content posted there in the same form as the filter rules for content they want to avoid.
The problem would of course be quality of tags if you have a complex tagging system and rely solely on the author for tagging. That is how I arrived at the idea of letting third parties add tags, similar to e.g. sites like e-hentai.org or image hosting sites like rule34.xxx (both obviously NSFW) where third parties can add information on language, author, art style, characters, kinks,…
Outside the porn focus one could have similar tags for news about a specific country or company or website (e.g. news about reddit or twitter) or political party or NGO or movie franchise or show or fictional universe, for the whole set of pet attributes and actions reddit modelled by having hundreds of cat related subreddits,
Of course something like that would likely require one of those tuple tag systems (language:english,artstyle:rendered,…). Possibly even something more complicated depending on the possible use-cases for filters.