[jvoorhis]

You're answering the wrong question. They're asking about passphrase for SSH key not OS user accounts.

Adding a passphrase to your SSH private key is a best practice but not easily enforced without some manual process or command and control for sysadmin workstations.

Hardware tokens containing private keys and used via PKCS#11 are another option for providing some assurance. The token itself can require a PIN.

[brokenwren]

You are correct. I missed read that question.

If you select an easy passphrase for your key, it's similar to selecting a weak password for your bank account when your bank doesn't enforce password constraints. Buyer beware.

An ideal solution would be a two-factor key encryption. That would require the two-factor key to unlock the private key. There might be something out there like that. I haven't looked though.

[jvoorhis]

Physical cryptographic tokens meet this requirement: something you have-possession of the token itself-and something you know-the PIN to operate it.