The latest version of Chrome bakes Chromecast support directly into the browser, forgoing the need for the Chromecast extension. It appears that something in this upgrade broke support for TwitchCast. The dropdown to select which Chromecast still appears (though in its new form), however TwitchCast never recognizes when the Chromecast has been selected.
Really sad this is broken as regular twitch chromecast hasn’t worked for me in months. It finally has started loading again, but now the video stutters even though the audio is 100% fine. I don’t have speakers on my computer, so TwitchCast is really my only option to enjoy Twitch.