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For security keys implementing FIDO (which sometimes you can see referred to as WebAuthn, although that is partially inaccurate), there is currently no good way of backing up the key. If you lose it, then you'll need to recover your account with every service you had it registed it with, meaning that you'll need at least an additional factor on each service that's most likely not as secure as a FIDO key, unless you're registering two keys for every service which is very cumbersome and still makes you re register a lost key with each service.

There are proposals to address this either by chaining trust between security keys or by sharing "passkeys" (a webauthn credential). see https://news.ycombinator.com/item?id=31272867 Only apple implements it today as far as I know so there's no good way to recover from a lost or damaged key if you're not exclusively in the apple ecosystem