The Chromecast audio which allowed you to turn any old speakers with an AUX input into a wireless streaming device to play music from your smart phone is no longer in production. After just 3 years in the market it’s the end of the line.
With more an more audio systems coming with bluetooth streaming by default and the slew of audio assistant/speakers out there such as the Amazon Alexa or the Google Home line of productions the need to have a device such as the Chromecast Audio is becoming less and less and google decided it will no longer support the device.
My best bet is google will follow in the lines of Amazon Echo Input which attaches to regular speakers via a 3.5mm audio cable or Bluetooth and adds Alexa. I wouldn’t be surprised if we see google come out with something very similar but adds support for the google assistant. Mark my words!