Going forward, there is no function that will return the same value that "GetDeviceUniqueID" returns now. In order to transition seamlessly you'd need to be getting the current value from "GetChannelClientId" and switching them out or linking them up on your back end.
I don't know for sure what they (Roku co) will do, but I'm guessing the deprecated function will issue a warning in debug and not crash the app. But you won't be able to get a new app approved with that function in it.
aspiring