This is what I really want to know. I don't want to risk respawning some annoying hunter enemies or (god forbid) the Winter Lantern or the bosses without first knowing if it's even worth it - 3971 echoes apiece to be able to upgrade everything in one go would be really nice, like a bonfire ascetic system except the difficulty doesn't go up (unless it does?)
EDIT: it appears that no, things like coldbloods, shards, even things like molotovs do not respawn. But bosses and non-respawning enemies don't either. Just vials and bullets basically (and the sedatives you pick up below the ceiling pig too). And that's not always consistent. There were spots I knew were vial or bullet pickups that had nothing.
One I'm unsure of is the bold hunter marks (there are two you can pick up besides the ones Eileen gives you), I didn't check that. But anyway I reset my save since it was a big waste of echoes if you ask me. I have seen you can repeatedly give echoes to the bath. Does it do anything more if given multiple offerings or is it just the same things resurrected after you pick those up? What an incredible disappointment this was. Mysterious 7 insight door to a bath with more mysterious properties wanting 3971 echoes (extremely specific number there) that does something. And it's just bullet and vial pickups you could just grind enemies for or purchase. I don't see the point to this.