It's not structural but the cleat will wear a lot faster without it.

Shimano appears to have changed adhesives/plastics used to adhere the yellow bumpers to the cleat body. My first pair of cleats back in 2012 lasted about 10,000 miles and at the end the yellow bumpers were still attached and very worn.

More recent cleats have had the bumpers fall off within the first 500 miles. You can clean the contact areas, scuff with sandpaper or a file and use Shoe Goo or epoxy to reattach but this often doesn't last very long.