Should I use BIP39 passphrase to protect my seed phrase?

No. Passphrases are a single point of failure.

Why you shouldn't add a passphrase:

A BIP39 passphrase, or a 13th or 25th word, is commonly used with singlesig wallets as an easy way to add a layer of security. However, if you forget or lose your passphrase, you no longer hold the key to your bitcoin, even if you have the original seed phrase card!

While we understand the desire for extra security with securing your bitcoin, adding complexity can actually be a security threat itself. We have found that in many cases, clients lose access to their funds not because of a lack of security but because of having too much!

Additionally, passphrases make inheritance much more difficult. If your heirs don't know your passphrase, they will be unable to access your bitcoin, even with Unchained's help.

For these reasons, we do not recommend using a BIP39 passphrase in combination with multisig. Multisig already adds ample security and eliminates single points of failure.

Passphrases on Trezor:

Trezor hardware wallets have the passphrase option enabled by default. Trezor will ask you if you want to use a "Standard wallet" (no passphrase) or a "Hidden wallet" (with a passphrase). Our Trezor setup guide, recommends turning off this passphrase option completely.

In Trezor Suite, click the gear icon for Settings in the left-side menu, followed by Device near the top of your screen. Scroll down to Passphrase, turn off the switch, and confirm on your Trezor.