Per TIP-078, revoke minting permissions on the first two deprecated thUSD contracts as a security measure:
As an additional security measure, the DAO should revoke Mint authorization for the collaterals on the deprecated first two thUSD token contracts. Authorized contracts to mint thUSD are the old borrowerOperations contracts for both collaterals (tBTC and ETH) of each deployment set.
Please see forum link for further details.