Actually, I named it incorrectly. I treated as private collections those that don't appear in the listing, but you can still share them with friends. I changed it so that they are truly private, and I will add another status someday - non-public.
I understand now, I'll think it over during the weekend. I haven't really considered it before because the fediverse is still a relatively small and close-knit community imo. Excluding a specific instance would likely involve PR proposal to Lemmy's repository, but that's fine. Thanks for the feedback.
I don't understand how content grouping can be used for potential abuses, to be honest. Collections are related to the user, so it will be subject to instance moderation in case of such an attempt. The community can also switch to private mode - then this content will not be aggregated in collections. Moreover, from what I know, there are already external tools that do similar things.
This is just a prototype, there will probably be some errors to fix ;-) But I want to see how it performs in practice and over time, I will refine the whole thing, and that will take a bit more time.