iPhone has never had split screen. Maybe there's a jailbreak tweak for it. Only iPad can do split screen.
I could see it being useful on iPad (never tried), but I can guess that on iPhone it hasn’t been implemented because for the vast majority it’s not a needed or useful feature. With the screen real estate of an iPad I’d probably use it, but in all the years I had an Android I maybe used it 10 or less times because I’d end up exiting out - more navigating around with two apps open. But that might have just been my particular use case.