We're using Firebase for our back end which requires either an email or phone number for the user. However, if you want to be sly you could just put in another username in there and it should/could still work (don't hold me on that). Just means we can't contact you with email so push notifications are hopefully turned on.

Firebase has anonymous accounts which you later can connect with a new login provider (password, FB, google, etc) to provide persistent login.

While this is true. Name one mainstream app that actually does this or at least doesn't replace it with the (way worse) phone number option?

While there are many bad examples, that should not be a reason not to do it right ;)

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