You still can- Lowes carries Craftsman now and honors the forever warranty (no receipt needed) for tools purchased from other retailers. That said, if you have older USA-made Craftsman stuff I don't recommend warrantying them unless they're totally destroyed as you'll be given a modern cheaply-made Chinese tool. If it's something serviceable like a ratchet, better to buy a kit to repair it rather than hand it in.
Craftsman has always been rebranded tools. They used to do a good job picking well-made tools, but now they're just cashing in on whatever value is left in the brand.
They never made the tools themselves, yes, but they weren't exclusively re-brands. Rather, their tools were a mix of in-house designs (manufacturing outsourced) and re-brands.