Vlad I think the concept of a lower limit is the privacy preservation in itself. Up the limit to increase certainty. If say 50+ Kagi users have boosted the same site, it can be reasonably safe to to say that the website in question is publicly popular enough to not be something that need's to be kept secret especially considering that websites themselves are publicly accessible. So no to the opt-in mechanism as long as a good minimum to be shown publicly exists.
To rephrase, this suggestion is just a dedicated page found somewhere like Kagi.com/TopRanks that is bare bones simple and extremely useful. It would look very similar to this.
Kagi.com/TopRanks webpage is separated into two sides. On the left side it shows exactly like this:
Kagi's Favorite Boosted Sites
News.ycombinator.com (1043 Boosts)
MakeUseOf.com (543 Boosts)
On the right side of the page it shows Blocked domains:
Kagi's Most Hated Sites
CNN.com (1051 Blocks)
FoxNews.com (953 Blocks)
As you can see no info from who those 1051 users who blocked a specific site are.