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.