Google announced on Twitter that Stadia can now be played on any Chromecast Ultra.
Previously, the only Chromecasts compatible with Stadia were the ones that shipped with the Stadia controllers. in either the Founders or Premiere bundles. Stadia is still not compatible with TVs that have Chromecast built-in. In a follow-up tweet, Google said it will add more compatible devices in the future.
Source: IGN.com Google Stadia Can Now Be Played on Any Chromecast Ultra