It looks like Samsung removed it, but you can follow these steps to try and boot into the Galaxy S8 Fastboot Mode (aka Bootloader Mode).
Here you will find all of the Galaxy S8 and Galaxy S8+ tips, tricks, tutorials and video guides to walk you through the phone from start to finish. This includes everything from system modifications like gaining root access, all the way to just learning the ins and outs of the software Samsung provides.
Be sure to bookmark this page as it will be updated daily with new articles to help you learn how to use your new smartphone. This includes very detailed and simple step by step instructions as well as walk through videos so you can follow along with me. Each Galaxy S8 tip will also come with an explanation at the bottom so you know why we’re doing something, what each step actually does, and how to change things back to the way they were before we started.
Please feel free to leave comments on the page you’re looking to receive help for and I will do my best to help you if I can.