According to a post on the YouTube Blog, “YouTube Music is your personal guide through the complete world of music—whether it’s a hot new song, hard to find gem, or an unmissable music video. Music fans on Android phones can now easily unlock the magic of YouTube Music, which will come installed on all new devices launching with Android 10 (and Android 9), including the Pixel series.”
That being said, it does not mean that Google has done away with Play Music. In fact, users who want to install Play Music back on their devices in favor of YouTube Music will be able to do so. All you need to do is go to the Play Store and search for the app and you can redownload it once again.