I'm betting they probably sell them to a bulk reseller of which there are many. If you're looking to buy from one they're out there but may or may not be looking for small customers.

Physically shredding.

https://youtu.be/TQoKFovvigI?t=98

Physical destruction. Destruction of decommissioned drives by shredding, crushing, or passing a bolt through the drive is a standard practice for security-sensitive operators. Some IaaS operators I've worked with extend this to every component of systems that could ever hold customer or proprietary data in memory.

It's hard to 100 percent know your keys haven't been stolen. They are very small things, easily smuggled out in an ICMP packet, and must be in RAM of whatever machine the disk is in.

For many businesses, crushing the disk makes better business sense than take the risk that they have historically lost encryption keys and are now about to hand their data away to an attacker.