Web Components, including Shadow DOM, are accessible because assistive technologies encounter pages as rendered, meaning the entire document is read as “one happy tree”. With a polyfill to add broader support, users with modern browsers can interact with Web Components using a keyboard, touch or mouse. Screen readers will also read the page aloud, illustrating how Shadow DOM fits seamlessly into the regular DOM.
It’s easy: to make Web Components accessible to all users, write accessible code.
Read more about Shadow DOM in this article from 2014.