Was it ever supported? The response in the forum is "[a]t this time, Chromium is not supported for use with Google Cast." which doesn't explicitly say that support was retracted. Original title, lest it changes: Chromium is no longer supported for Chromecast (productforums.google.com) 3 points by keeperofdakeys 1 hour ago
I don't have a Chromecast set up, so I'm not 100% sure on this, but my understanding is that Chromium users could cast using the Google Cast extension, and that Google did something (intentionally or unintentionally) to break the Cast extension and didn't fix it because casting is built into directly into Chrome now.
And it's generally believed that Google moved the Chromecast-functionality into Chrome and away from an extension to not necessarily lock out Chromium, but rather to lock out other Chromium-based browsers which could use their extensions, so for example Opera.
Firefox and, I believe, Edge are also getting support for Chrome extensions in the foreseeable future, so this might have increased the incentive to lock out browsers that can use their extensions, too.