What is this backup passphrase? Is it another pass phrase that can be used to unlock the original mnemonic seed phrase for your wallets? If not, what does it serves as in wallet security?
A passphrase is an extra layer of security for your wallet, it is an extra 'word' added to your seed phrase which creates another wallet and keys for you, take note that your passphrase does not have to be a 'word', you can use and add whatever you want, even characters to make it stronger and hard to brute force. So whenever you use your seed phrase without adding that passphrase you get the base wallet, but when you use your seed phrase + passphrase you get your second wallet, this set up is great for plausible deniability.
So with a passphrase set, if an attacker compromises your seed phrase, they will still need the passphrase to access your funds. Take note that if you use a seed phrase + passphrase set up, you will have more backups and you have to keep them in separate locations to avoid the possibility of a single point of failure.
It's also very important not to lose it as well. Most people focus way too much on the security side of things but then they forget about the issues that they have on their hands, which is remembering it, you will probably not remember it or even if you do, you may eventually forget it. While trying to secure it carefully, you may end up locking yourself out along with everyone else and that would result with something that would not be that much a profitable business.
This is why I believe that we need to do a lot better, and that could end up reaching a little later than expected as well. I understand that we could do much worse but we could end up trying to do the best thing we could potentially do by saving it somewhere easy to access by us, but nobody else.