I hope some point in the future Apple untie Safari from macOS and iOS so they can be updated independently without having to wait months to operating system updates to come out..
But they aren't backported on iOS, I assume purely for marketing (look at our new tab layout! you'll get it in the fall), which doesn't make sense considering the last time anyone cared about a browser feature was probably when tabs were added to IE7.
Take Chrome for example, where there's roughly an update every 60 days and the option for them to push out security fixes quicker.
Imagine having to wait for an operating system update (major or minor version) to just get the latest secure version of Chrome. That's pretty much where Safari is, especially on iOS.
Don't get me wrong, I think Safari gets a lot of unwarranted hate and it can certainly be a bit quirky here and there, but if they didn't have to wait for the next major/minor release of macOS/iOS to update Safari I think it'd be in a much better place.