Epic Games has silently enabled 60 FPS for the iPhone XR, XS and XS Max in its latest update for Fortnite on iOS, according to the Senior PR Manager at the company. The update (v. 6.31) is live now on the App Store.
https://www.notebookcheck.net/Epic-Games-brings-60-FPS-Fortnite-to-the-latest-trio-of-iPhones.373716.0.html