It would be abused into the ground by the same people who set up these scam sites in the first place. For most things on the internet, the crappiness of the average person is the limiting factor.
Idk why everyone is praising public anything, because local communities and knowledge webs worked fine pre-internet. Most of the bullshit came with globalization.
But it's probably too privacy sensitive (if I see which sites you upvoted, then I have some information about you). Hence, that's probably why this has to be either completely private or completely public.
But yes, it would be nice to have this in a less proprietary way. But I fear it's either going to be a privacy issue (because the person you trust has to publish every page they like and dislike), or it's going to be anonymous and therefore easily gamed by bad actors.
For example, I wouldn't mind sharing my upvoted websites/videos/products with everyone on HN, as long as it is anonymized. Bad actors can be distrusted by the community, I suppose (though moderation seems to be largely an unsolved problem still).
Click/dismiss "make it private/public" bubble.
Set default vote visibility in settings.
Enable/disable the bubble.
List/add/remove sites in a privacy list.
But the common notion is that it's probably too hard for "regular" users, so we have the internet of nonsense instead. Few more years and we won't find anything good.
If you spend 5 minutes on it, can you think of a way? If you can, congratulations, now imagine millions of other people thought about the same tricks and you get the reason why Google can't ever really win against SEO.
Eg if they can get to a place where SEO efforts point in the same direction as making your website genuinely more useful to people, then that's good enough for Google.
Just cluster people by downvotes and whatever other thousand metrics are already being tracked, and allow them to see results given by other clusters by showing which areas are dense on a PCA or something or saying 'I want my results to only be influenced by people who have downvoted github.org' or whatever.