De-authorizing devices on Google Play Music is simply like removing a device on Play Music app.
A few months ago, Google reintroduced its limits on the number of devices for Google Play Music subscribers who could de-authorized each year. Before this, users could remove mobile devices from their ten-device bank at any time. But since 7th August onwards, users were told that they could now de-authorize their devices to just four per year.
Now it seems that the de-authorization limit may have been lifted or at least temporary eased. Several staff members from Android Central have found that they’re able to remove more than four devices. Specifically, Google Play Music on the web allows users to de-authorize four devices but the Play Music app on both Android and iOS seems to give users limitless de-authorization.
It’s unsure if this is an intentional change by Google. It’s also possible that this is merely a software glitch. In any case, if you’ve got an old, unused phones taking up valuable device slots, now might be a good time to see about de-authorizing them.
Source: Android Central