You can keep track of which browsers support async (and defer) in the MDN website here:
https://developer.mozilla.org/en-US/docs/HTML/Element/script

The async is currently supported by all latest versions of the major browsers. It has been supported for some years now on most browsers.

You can keep track of which browsers support async (and defer) in the MDN website here:
https://developer.mozilla.org/en-US/docs/HTML/Element/script