It's not even much data at all. A game dev friend of mine took a look at what is shared through Steam's own API, and it's just a unique ID basically verifying that the steam account belongs to that steam user. The worst they can do with that is ask if the user already owns this or that game, likely just from the same publisher.
They probably just want the ability to better ban people if they are griefing other players (or make sure people can't cheat their way to more super credits)
The whole thing is such a nothing burger from sony's part, it makes it even more absurd that they're doing this!
Depends on the kind of collectible and the kind of game it is, I suppose. Collectibles for my house in Monster Hunter is awesome.
Feathers in Assassin's Creed is not.