I don't think there is an answer to this to be honest - just how likely the agency is to be sued. Everything is designed (OK some things evolve, but we won't get into that debate...). So everything that is reasonably new has been designed by someone who owns the copyright to it. Some things are designed by famous people or companies that make the effort to enforce their copyright. Much other stuff is just made by the millions and the company/designer is simply interested in producing the product at low cost - they may stop someone copying the product, but a photo of the product is so far below their radar that they don't care, and if they thought about it, they may think it could help sales.
So, following that logic, everything that has been designed needs a release, but 90% of those items have been designed by people/companies who don't care. I think Shutterstock and the other agencies are trying their best to accept what has a low chance of dispute and reject the high risk items.
I had some 30 year old plastic snowmen rejected for property release, but they relented when I told them how old the items were...
Steve