Updated my paper:
https://news.ycombinator.com/item?id=37833390
Scenarios dealing with the loss of Passkeys:
The scenarios for dealing with the loss of Passkeys are effectively the same as dealing with the loss of your Password Manager (if you use one) or otherwise stored passwords.
Dealing with the loss of all your devices that use Passkeys
If you manage to lose access to all your devices that are used to authenticate via Passkeys (e.g., a house fire), then there are two main outcomes: either you have your Passkeys synchronized to a cloud provider or other external entity that still has a copy of all your Passkeys, or you do not. If you do not have a backup of all your Passkeys, they are gone, and you will need to fall back to account recovery for each affected account. If you have a backup of your Passkeys, you would need to regain access to it on a new device and then synchronize the Passkeys to it and use them as normal.
Dealing with the loss of your accounts that synchronize and store Passkeys
If you use a synchronization service attached to an account, it is possible that the account can be deleted or access to it otherwise lost. In this event, you would most likely still have a working copy of your Passkeys on your devices, and depending on whether or not you can export them or reconfigure synchronization with a new account, you would be able to add them to a new account, effectively creating a new account to store and synchronize your Passkeys.
Dealing with the loss of all your Passkeys
If your Passkey account is not only deleted but also tells all your devices to delete the Passkeys, or you lose all your devices and the accounts are deleted due to inactivity then you are basically in the same situation as having lost all your devices and not having a backup. You will need to fall back to account recovery for each affected account.