iOS…iPadOS…are you getting it? These are not two different operating systems…
The above screenshot is from the iPad, and it's where you get to choose how you would like apps to display in the OS. Given my recent assertions that iOS and iPadOS are already the same thing, but with different exposed features on some devices, I'd suggest that the iPhone of today effectively runs the "full screen apps" mode. There's no split screen, picture-in-picture is here, and you can swipe across the bottom of the screen to quickly switch between recent apps. That's how it works on the iPhone and the iPad when using this mode.
Windowed apps and Stage Manager are effectively two variants of the same windowing system, and while these are not exposed to iPhone users, we do know that they are in the code running on your iPhone right now.
Based on reports from Mark Gurman, including this latest, it sounds like the folding iPhone will have a split screen view when unfolded, but no free window management.
My expectation
I expect that in iPadOS 27, these 3 options will remain, but the full screen apps option will bring the return of split screen that does not involved free-floating windows like we have now in iPadOS 26. In June, this will make a lot of iPad users happy who didn't love needing to opt into full windowing to get the split views they used to love.
However, this new full screen apps mode will also come to the iPhone Fold this fall, allowing those users to run apps full screen and with a more iPad-style layout, while also allowing side-by-side apps on the internal screen. The iPhone would not need free floating windows, so I would not expect that to come to the Fold.
Apple could keep the naming of the OS different on the iPhone and iPad, but if this prediction comes to pass, it really does feel like in 2027 they could unify the OS naming across all their phone and tablet devices, but we'll see…that stirred people up last time I suggested it…