This is a hard one to judge. But, given that it’s currently not a default setting, and not  advertised heavily, it’s not likely that many buyers are using it yet.
Most buyers come to eBay with a set idea of how to search for products. They enter a search term, check the results and then consider using filters. Many won’t look beyond the filters that they are already familiar with. There’s a kind of learned blindness when it comes to shopping online!
The Guaranteed Delivery button should not be compared with the Amazon Prime checkbox. People sign up and pay for Prime, then are naturally drawn to the Prime badge. Many shoppers report automatically ticking the box because they have signed up to Prime and want to get the most out of it.
eBay Guaranteed Delivery is not like that. It’s not a program that buyers sign up to, and they don’t pay a membership fee. Most of them won’t know what it is or even realize that it exists.
Some, of course, will notice the new option and give it a try. If there are plenty of eligible products and buyers learn to trust the delivery times, usage should grow organically. If eBay are satisfied with seller performance and buyer feedback, perhaps they will push it with a major advertising campaign during a peak selling period. Then it could really take off.