Well, now you can’t.
You can. You just need to de-select your old device and set the account up on the new device. I add Netflix to new devices all the time. It's just a pain to juggle between them with log-in codes.
For folks who aren't as tech sophisticated, this is a royal pain in the ass. But then so is phone streaming.
Just your standard corpo greed.
I'm willing to bet the dropped feature stems more from the need to invest company resources in support for casting from the phone to begin with. Why bother doing that when modern TVs are all "Smart" anyway? Just throw the QR Code on the screen and let the single device to all the work.