Caniuse shows that IntersectionObserver API was added to iOS 12.2 in March 2019.
If those were tested on iOS too, then it also proves it should work - all browsers on iOS use WebKit under the hood.
@MarcP, have you, by any chance, disabled IntersectionObserver in Settings > Safari > Advanced > Experimental Features? It should be enabled by default. (see Safari Technology Preview Release 71 notes)