Google has added a network-level toggle for cast notifications on your device, so now users can disable the notifications that show up when casting media content to a Chromecast or Chromecast Audio. If you have a Chromecast device in your home and you use it often enough, or someone in the home uses it often enough, then you've likely noticed the notifications that populate in your status bar when something is being cast. These notifications also act as controls for a simpler way to manage the media that's playing, allowing users to pause and play the content as well as exit it and skip tracks or episodes. They also show up for all users who are connected to the same Wi-Fi network that the Chromecast is connected to, which might annoy some users if they aren't the ones using the Chromecast in the first place.
Now that the network-level toggle is being implemented users can turn these notifications off for the entire network which means the owner nor any other users connected to the network will see these notifications in the future even when media is being cast to a device. If you didn't care for those controls then this is likely a welcomed change, though it could understandably cause confusion and annoyance for users on an individual level if someone in the home prefers these controls to be there.
The nice thing about these cast notifications is that they're persistent and they tend to stay right where they are until you dismiss them. On occasion, the standard notifications for Netflix and Hulu or other media apps will disappear on their own when the Chromecast is still being used, which makes it hard to pause content if the controls aren't accessible. These new cast notifications were a nice fix for those issues. If you do want to turn them off, you'll have to wait til the feature is actually live as it doesn't seem to be showing up yet, but once it is pushed out to users it'll be available in the Google Home app. Once opened, tap the devices icon in the top right corner and find the card of the device you want to toggle the notifications off for, tap on the menu for that specific device, and disable the option that lets other users on the network control a cast device, which will be listed under the Device Info section. That's it, so at this point it's just a waiting game for Google to flip the switch.