From Google:
"We’re gradually going to shut down Google Play Music, the Music Store on Google Play and Music Manager soon, with everyone losing access in the months to come. You can read our full announcement here. This announcement follows the launch of our new migration process (back in May) which lets all Google Play Music listeners easily transfer their data and make the simple switch to YouTube Music"
Interestingly, no mention of the new requirement to pay $9.99 pm to listen to your own music in the background, or the fact that ads will play on previously purchased tracks without becoming a premium subscriber.