Smartcards e.g Yubikeys help with this. You store the key on the smartcard, then use ssh-agent to have the smartcard perform whatever cryptographic operations a

Smartcards e.g Yubikeys help with this. You store the key on the smartcard, then use ssh-agent to have the smartcard perform whatever cryptographic operations are necessary. On Yubikeys at least, the keys can never leave the smartcard, by design.