It seems Google has reimposed the limits on device deauthorizations for Google Play Music that were originally put in place a couple of years ago, before being lifted shortly afterwards. The Play Music settings page — and Google's support docs — now inform users "you can deauthorize four devices per year"; go over that limit and it'll tell you "sorry, you've deauthorized too many devices." When that happens you'll need to wait, potentially up to a year, to remove devices from your account and stop them counting towards your 10 device limit. (Max out both limits and you're effectively prevented from streaming Play Music on any additional mobile devices.)
The change means you'll need to think more carefully about which devices you authorize, and how you spend those precious four deauths each year.
The deauthorization limit was unpopular in 2012, and it's sure to cause a few headaches this time around too. It makes life tricker for anyone using a lot of different devices to do their job, such as tech journalists who review phones, and many others working in the mobile industry. We've reached out to Google to find out what's going on, and we'll update this post with any new info as it arrives.