A render made by a case manufacturer showing an iPhone in their rugged case appears to show the iPhone 8 will have an all-screen design and an on-screen button. The location of the fingerprint scanner isn't shown, but it could be under the screen, on the back, or completely absent in favor of an IR scanner.
https://www.notebookcheck.net/New-iPhone-8-case-render-adds-support-to-all-screen-design.240468.0.html