New Macs keep asking about old discarded Mac credentials

Solution 1:

This happens due to you having iCloud Keychain enabled. The Keychain is end-to-end encrypted so that Apple cannot decrypt the contents of your keychain on their servers.

When you add a new computer to your iCloud account, it tries to download the keychain to that computer. The contents cannot however be accessed with just the iCloud password - you will need to give the passphrase for the old device. Even though that device is really not around anymore, it was used to protect the Keychain originally, so its passphrase can help you unlock it now.

Note: The Keychain is not somehow "permanently" linked to your old computer. Now that you have started using your new computer, you do not need the old computer's passphrase anymore. If you choose to add new devices later, you will be asked to enter the passphrase of your new computer instead.

Solution 2:

Could it be that you forgot to sign out of iCloud on the 2019 MacBook Pro from your former job?

When setting up a new device, if iCloud Keychain is active, you will be asked to type the login password or passcode of another device that you are signed in to with your Apple ID, see Approve a device to use iCloud Keychain for details.

If you forgot to sign out of iCloud on one of your devices and have given the device away, it could happen that Apple asks you to type the password of that device. In this case, you will have to manually remove the device, as explained in this support article: https://support.apple.com/en-us/HT205064. For example, using the Apple ID website:

1. Sign in to your Apple ID account page (https://appleid.apple.com), then scroll to Devices.

2. Click the device to see its details, then click "Remove from account."

3. Review the message that appears.

4. Click again to confirm that you want to remove the device.

For more information on how Apple designs security, see the Apple Platform Security website.