Any work I have ever done as a freelancer, and any consultants who have ever done work for my company all designs/IP belongs to the client - with the IP/ownership rights transferred upon payment.
If someone had a contract that said they own the rights to anything you “don’t use” that would be a very subjective statement in court. For example, what if I didn’t choose to use your design, but 3 years from now dug it up and decided it would be great for another product, only to find out that it’s now on the shelves as something else? Would be interesting to see how anyone who uses that actually phrases it, and if their contract has stipulations specifically outlining under what circumstances that’s OK.