Shadow DOM v0 is deprecated at M70, and removed in M80. All code has been deleted as of M89. Intent to Deprecate: https://groups.google.com/a/chromium.org/d/msg/blink-dev/h-JwMiPUnuU/sl79aLoLBQAJ Shadow DOM v0 was an experimental technology, never implemented in any other browser. The spec is superseded by Shadow DOM v1, which is widely supported. Note that the /deep/ combinator, which was changed to a no-op in M63, has been completely removed in M89.
Shadow DOM V0, Custom Elements V0, and HTML Imports were launched in 2014, but they did not get adopted by other browser engines. Instead, Shadow DOM V1, Custom Elements V1, and ES modules are widely adopted by various browser engines today. Chrome has shipped Shadow DOM V1 / Custom Elements V1 in 2016 and ES modules in 2017.
Docs: https://developers.google.com/web/updates/2019/07/web-components-time-to-upgrade http://www.html5rocks.com/en/tutorials/webcomponents/shadowdom/ https://developer.mozilla.org/en-US/docs/Web/Web_Components/Shadow_DOM