This meta bug will cover multiple stages of transitioning users: * Encouraging single-device users to switch. Most of these users are misunderstanding Sync, and will be better served by FxA Sync. * Encouraging multi-device users to switch. Any additional help we offer here, such as automatically transitioning your second and other devices, falls under Bug 956444 and its desktop counterpart. * Giving less-gentle nudges as we get closer to client EOL. Note that server EOL messaging is covered by Bug 895518.
N.B., we expect the number of Android single-device users to be very small indeed (~0), so I don't think it's worth addressing those users.
This is a meta bug for migration with all blocking bugs closed, so calling this fixed.