This is not true. We still use my iPhone SE with a 4" screen as the primary testing device for our iOS app - https://apps.apple.com/gb/app/vendi-buy-sell-verified-phones...

There are many advantages to it. If it looks good on the SE, we can be reasonably sure it will look good on everything else (though this was affected by the introduction of the notch). Also, flagship iPhones can easily handle badly designed apps but my phone will show lag and artifacts while scrolling through an infinite scrolling screen.