It seems as though you are now able to deauthorise as many devices as you want from your account. I've managed to get rid of all my old devices which is great as I didn't have any slots left. I'm not sure how long it will last for.
Has Google Play Music's deauthorization limit been lifted? http://www.androidcentral.com/has-google-play-musics-deauthorization-limit-been-lifted
Has Google Play Music's deauthorization limit been lifted? http://www.androidcentral.com/has-google-play-musics-deauthorization-limit-been-lifted