I, too, hate the inability to hide the formatting toolbar. I'm on an iPhone SE, prefer my tiny phone, and hate clutter. If I want the toolbar, I'd like a way to display it, if not, it should not eat up my tiny screen. I have literally 1.25 vertical inches of text (9 lines at my display size) visible between the header bar and the formatting bar/keyboard. It's very annoying. And there is no pinch-shrink the keyboard on iPhone SE. Maybe that's an iPad feature. Would be way too small on iPhone SE.
So, Apple, PLEASE remember small screen users and give us back more vertical message space. Even two more lines of text would be better than permanent display of a toolbar I rarely need, but header bar could also be shorten a bit, and there are at least 5-6 additional mm above the message window, below the signal bar, that could also be leveraged.
I'm hoping for a still-small iPhone SE2 at an affordable price, though it now sounds like the next iteration of the SE may be based on the iPhone 8 chassis. Well, I will hope until the end!