matteocontrini you will need a new search gambit. The implementation should be easy, as there's a comment_count column you can use.
You can take a look at the is:hidden gambit https://github.com/flarum/core/blob/master/src/Discussion/Search/Gambit/HiddenGambit.php It's even already using the column you need.
Basically create a new gambit like this one with a different name, remove the hidden_at stuff and change the comment_count comparison from 0 to 1. You can then register the gambit via the Flarum\Event\ConfigureDiscussionGambits event, and I think that's it.
With that you will be able to type the name of your gambit in the search. If you want a new page that lists them, you can see how the Subscriptions extension creates a "Following" filter as a page, or as you said, the Tags discussion does the same (it might be a bit more complex in the Tags extension).
