Seed Stacks are made of BIP Keys.  BIP Keys are made by stamping a key on a BIP Coin.  BIP Coins are M8 x24mm washers. 

After a wallet that has been backed up onto a Seed Stack is no longer used for one reason or the other, the BIP Keys are no longer secret or confidential; but their value isn’t lost.  Once a BIP Key (a BIP39 dictionary word and key is stamped into a washer) is created it will persist in the environment.  Much like a coin.  Whether the previously used BIP Key finds itself in the bottom of a junk drawer, tossed into a stream or a fountain or bouncing around the bottom of a tool box, it is no longer of use to the owner and can be collected, traded, gifted or sold.  Theoretically, a BIP Key tossed into a well would likely be found some day in the future and likely will still be usable to anyone who happens to have a wallet that uses that same Seed Word/Key combination.  Granted, it would be rare to find a BIP Key that matches your wallet, however, I personally have had two different wallets that used the exact same word in the exact same (first) position.

BIP Coins on the other hand could theoretically be made on demand or made at random.  Anyone could sit down and stamp BIP39 dictionary words on washers and sell them.  This would remove a lot of the work for someone who does not have the skills or ability to stamp their own.