No tether has ever been destroyed. While tether in its terms maintains that Tethers are redeemable "provided that you are a fully verified customer of Tether" [1], there is no known case of this ever happening, in fact it is a running joke that the tether signup page [2] is permanently suspended. Nor does anyone know who is buying them (and causing their creation). For example, when $250m of them were printed in june [3], which customer was it that deposited $250m USD? Or did they print them out of thin air? Nobody knows for sure.
But if they are in fact backed 1:1 with USDs, then it raises the question: if the tether peg is slipping below $1, why doesn't Tether or one of these alleged "verified customers" buy them up at a discount, redeem and destroy them for USD, and bank the difference?
The answer to this question is left as an exercise for the reader.
[1] https://tether.to/legal/
[2] https://wallet.tether.to/app/#!/signup
[3] https://omniexplorer.info/tx/bd9520b9aea701e9606ad8a8f4d6852...