Grids are most useful in wider viewports (tablets and laptop/desktop browsers).
Except for IE11, basically all browsers that don't support Grid are outdated mobile browsers (pre-iOS 10.3 versions of Mobile Safari, Android Browser in older Android, and browsers that are based on Android Browser and have high usage in developing countries, like UC Browser)
But then the question becomes: can you (sanely) gracefully downgrade on those small screens (hopefully without making an entire early 00s separate "mobile" site)?
Except for IE11, basically all browsers that don't support Grid are outdated mobile browsers (pre-iOS 10.3 versions of Mobile Safari, Android Browser in older Android, and browsers that are based on Android Browser and have high usage in developing countries, like UC Browser)