UI elements that appear after the page is loaded can be annotated with
aria-live so that screen readers will read the changes in the UI.
— Mozilla Developer Network, retrieved 2019-09-03
It is up to the implementation to ensure the Material Design guidelines are used in a way that is accessible for all users. For example, a Material Design framework for Angular specifically addresses accessibility for snackbars in their implementation. They mention:
Snack-bar messages are announced via an
aria-live region. By default, the
polite setting is used.