If you no longer have access to a device with imessage contact key verification turned on, you can reset it from the web As for the fn (function) key, i haven’t found a way to reassign or customize it, but so far, i haven’t needed it If you don’t use features like dictation or emoji shortcuts, you probably won’t miss it either. Do you really mean a security key Or do you mean a recovery key If you lose all of your trusted devices and security keys, you could be locked out of your account permanently. the information below is based on apple's support article
Set up a recovery key. Go to the security section and click edit Remove the device you no longer wish to use to verify your identity If you have additional devices with find my iphone enabled, you can verify them as trusted devices You can also add additional trusted phone numbers if you lost your recovery key
The automated call system continues It may ask you to press a key again By default the iphone does not send the keypress To fix it do this Change the option from banner to full screen However, when you generate a recovery key, you can't use account recovery.
Account recovery is typically used in the following common scenarios No access to trusted phone number or changed number
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