Since collections a product belongs to aren't serialized on the product resource, products/update would not be dependable in determining changes to what collections a product is published to. The same is true in that products belonging to a collection are not serialized on collections. While collections/update would fire, it would not give you specifics about what product was added or removed from it.
The answer here I feel would be to listen for collections/update to get the ID of the collection, and then make a GET request to `/admin/collects.json?collection_id=57943621688` to see if there are any new or missing collects that were there before (you'd probably have to track the state of this list from your end.
Hope that helps a bit.
Just following up on this as well.
The last reply from Shopify made it sound like it should work - i.e. we should be getting a trigger when a collection is updated - but that's not happening.
I'm not concerned about the hook not containing the details about the change - happy to fetch the products belonging to it when the hook is triggered. But I'm getting neither product nor collection hook triggers when you add a product to collection.
Just to be clear here, I'm talking about manual collections - for automated ones, the hook does get triggered because the collection object itself changes.