well, they no longer produce the "display-only" Chromecast in favor of their "Google TV" sticks with Remote etc. Not that much of a shock here, the market moved on from simple wireless display dongles. Unfortunately no sign of Google Cast protocol being opened for general purpose use. Would be great to be able to run your own custom Receiver-device without needing a Google certificate...
Open Screen Protocol exists and is very similar. It works and you can use it today (via the one and only reference implementation in the Chromium source tree)!
It's even pretty good & makes sense!
This was kind of part of the bargain for adding Presentation API to the web back in 2014/2015. Your site can itself trigger Chromecast! If that's true, then it seemed clear there should be a standardized way to talk to devices too, otherwise this wasn't really much of a standard. The same front-side/back-side happened with Web Push API for web sites which lead to the creation of a Web Push Protocol backend for actually sending push messages to the browser. It's not perfect but so far the web has somewhat stayed honest with APIs for the page having implementable backend protocols too. Presentation API sample (which oddly cant find my Chromecasts?): https://googlechrome.github.io/samples/presentation-api/
I really really wish there was some hardware support for this! I've been meaning to set it up locally & start using it some. Writing a native client seems not too absurdly hard.