Not surprised. Like chromecast audio, chromecast couldn't really serve an adequate amount of ads. Basically it's only value was it forced you to use stock youtube app to stream preventing any adblocking, but if you cast your screen, then it can't stop adblocks, so it makes sense to discontinue this product. There's some open source projects out there that might be worth looking into, NymphCast is one I saw, uses a rasberry pi.
You can just install bubbleUpnp on any android device and cast every app or website to your TV, including non-stock youtube (using it with Tubular, previously known as Newpipe).