I think it is not deducted from the total supply. It is still there although remained untouched. The total supply of Bitcoin is not affected. The 21 million Bitcoins will remain 21 million forever. Some of these have lost circulation though. Some of these are also forgotten forever. In fact, a significant portion of the total supply of Bitcoin is already lost in history for sure. Bitcoin has been used already for several years and certainly countless of private keys and passwords of personal wallets have been forgotten.