I respectfully disagree:
Different to what you expect and know is not a mess. It means you got a knowledge gap. However, UI wise, many new implementations are controversial nevertheless.
Stop ranting. Start learning.
Adding steps unnecessarily and making the system overall harder and more aggravating to use is a mess. That’s the problem here. For my workflow, SlideOver provided the perfect functionality for the apps that supported it.
Just this morning, I’m filling out a website form for work and need to reference a photo for information. In the past, I’d just invoke Spotlight, type “pho”, tap and drag the Photos app out, and then drop it to open in SlideOver, assuming I didn’t already have Photos in the SlideOver stack off-screen (in which case, drag my finger in from the right edge to open Photos instead). This let me interact with Photos and Safari simultaneously, with both apps always visible, and without Safari refreshing the page in the background. When done, slide Photos off the screen.
However, with the new windowing system (and Stage Manager isn’t much better, hence why I almost never used it on 18 and prior), the best way to do this is to pull out my iPhone and reference the photo there. The next best way I found is to shrink Safari to half-screen, open Photos, fling it to the open half, copy the relevant data, and then submit the form BEFORE I even think about closing photos and re-sizing Safari. I found that last bit out on the second try because at first I maximized Safari before submitting and the page deleted half of the information I had just entered. You can’t just pop a small Photos app out over top anymore because as soon as you focus on Safari, it hides the Photos app (any combination of apps really, these two were specific to my case). SlideOver cannot be properly emulated if you need to interact with the full-screen app while the smaller app is open, and I use SlideOver almost daily on my iPad.
My other bone of contention is the Preview app, but at least that one has a decent workaround by checking “Preview with Quick Look” on the relevant file types.
As for my interpretation of what people were asking of the iPad, the multitasking system (Split View/SlideOver alongside Stage Manager as choices) was never the problem. Most people wanted more powerful apps for the iPad. Final Cut Pro for iPad is plenty powerful enough for my needs, but some other people wanted the full blown Final Cut Pro on iPad (or Logic Pro, which isn’t on iPad at all). It wasn’t about the OS, it was about the apps. IMO, iPadOS 26 brings the worst of MacOS multitasking to iPad, made even worse by completely removing the simpler system that iPad used. I can learn to live with this, but all I’m asking to be happy with 26 is to give me toggles to enable classic multitasking on the Full Screen setting. The OS is already insanely bloated, so what’s another few hundred MB to keep the better system as an option?
(And yes, I’ve submitted feedback to Apple through Feedback).