Here an example from Sentry's master which other than bot triggered reverts are all verified: https://github.com/getsentry/sentry/commits/master/
Generate a long-lived root keypair (SC/C), the public key of which you add to the forge. You never sign directly with this. Instead you routinely generate new signing pairs. If compromised you hopefully only need to revoke the subkey so the blast radius is a lot smaller.
You could even do a three-tier one where you can keep the root key dead cold and literally lock it into a vault.
Last time I looked, this was not supported in GitHub, though; it only recognized signatures by explicitly trusted keys, not their signed subkeys.
This problem is largely solved in cryptocurrency-land. You have a hardware device that does the signing, which is recoverable from a 24 word seed that is stored offline (plus a passphrase which can be memorized or stored online so that it's not catastrophic if someone gets to your seed).
I just found out that Ledger actually supports SSH/PGP: https://support.ledger.com/article/115005200649-zd
To compromise a key you need to find a hidden piece of paper or engraved plate that your target has physically hidden somewhere. Plus guess a secret password (before your target has noticed you got to their seed and rang the alarm). Almost impossible to pull off.
I'm not sure what you mean about identity changing. If you mean a sex change or getting a new haircut, this is irrelevant to signing commits...
I fail to see how cryptocurrencies are in any way unique in this regard.