Early in August, Google upset the mob by changing their deauthorisation policy so that users could remove only a maximum of four devices a year. This caused a fair bit of discussion, but it appears that Google has quietly removed the four devices per year limitation – at least in their mobile apps.
Testing of the deauthorisation policy found that you can still only remove four devices on the web interface for Google Music, but once you hit that limit, you can still deauthorise devices from within the mobile app on Android. The Google Play Support page for Google Music still refers to four devices a year, saying ‘You can de-authorize up to four devices per year.’ but the practical effect at the moment is that you can now remove more.
You can check on the web in the settings page, or in your Google Play Music settings by going to the navigation pane and selecting Settings > My Devices.
If you regularly change devices or review devices and use Google Music, now might be the time to clean up all those unused devices.