Let's say you once worked in a company called Example Engineering Corporation, a very big and respected company. In fact, it is the biggest engineering company in the world, and it is a huge career accomplishment to be employed by it.
When you were an employee, you were given a corporate email, and you used it in Github so you could show everybody that you were working in the big leagues.
Thing is, you no longer are employed by Example Corporation. But to Github, that email is still valid, giving you an identity that you shouldn't have anymore.
So, should websites, specially the ones that can provide oauth authentication, ask you to periodically confirm if your email is still valid?