I won't use this as long as they're assuming that I have a cellphone. I have one now, but I can see myself without one soon as the future is internet calls, I mean I can make intentional video calls for free over wifi why would I use a cellphone?
I appreciate that they're trying to make our data more secure, just no this way. They might use Touch ID, or allow me to set it up like iCloud Keychain for instance.
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It will be a requirement for apps that access iCloud data from the server side

, like Outlook or any email app that needs access to your iCloud account.
For Apps that use iCloud to save/backup data or sync, it won't be a requirement.