Snackbars are not supposed to "interrupt" users but just quickly inform them. Any visual stimulant, such as Icons, could interrupt the user.
What's the difference between a snackbar and a toast?
Snackbars contain a single line of text directly related to the operation performed. They may contain a text action, but no icons. Toasts (Android only) are primarily used for system messaging. They also display at the bottom of the screen, but may not be swiped off-screen.
How many snackbars can be displayed at once?
Snackbars appear above all other elements on screen and only one can be displayed at a time.