If I buy a new iPhone 11 today how long should I expect it to receive updates for?
iPhone 6 is an outlier due to the 1gb of ram.The iPhone 6 shipped with iOS 8 on launch day. It's end of the line is iOS 12. Apple barred it from iOS 13.
So that's 4 years. But so what.
There's no reason not to continue using the final version it can run on for several more years after the upgrade path has ended. So it could go eight years.
But it's unlikely the hardware will last that long or you'd want to use it.
Even if the hardware lasts 4 years beyond cutoff date. Safari won't be aging as well on the newer web. Safari has always been the weakest link in iOS.
Reference in point: Safari on iOS 9 today doesn't work correctly for many website shopping carts and it's common for it to render some websites messed up with cut off pages or other weirdness