Prior to ChromeOS version 70, the notification and settings panels were split into two distinct panels. With version 70, Google changed this by merging them together. When the company made this change it also changed how clearing all notifications at once works. Before version 70, you would clear all notifications by tapping or clicking the notification stack icon in the notification panel. Android users are most familiar with this as it is the same icon on Android smartphones and tablets.
Now, in version 70 of ChromeOS, you clear all notifications by using the “Clear all” at the very bottom of accumulated notifications.
If you have a lot of notifications stacked, the “Clear all” button can seem to disappear. You just need to scroll all the way down to the bottom of the notification stack to locate it.