Why remove iPads split app feature no iOS 26?

It was not removed, it's just accessed differently and is much more powerful now.

Go to Settings ➡️ Multitasking & Dock and make sure either Windowed or Stage Manager is selected.

Then open an app, and swipe down from the top to bring down the menu bar, tap on the colored bubbles at the top, and then then press and hold on the green bubble and select an option from there.

No one here in this user-to-user forum can answer why Apple does or does not do something. Nor does Apple read here for feedback or suggestions.

You can let Apple know what you'd like to see here:

Product Feedback - Apple

iPadOS multitasking has received a significant makeover from that to which you may be accustomed in iPadOS 18.x and earlier - and may take a little time to learn and adjust your workflows. You would be well advised to review the official iPadOS User Guide and the many available online tutorials (such as YouTube) to become better acquainted with the new multitasking interface and associated gestures and controls.

Work with multiple windows at once on iPad - Apple Support

OS - iPadOS 26 - Apple

While the previous Slide Over view has been retired, Split View remains - although it is accessed differently. You can have two or three App windows side-by-side - or a spit the screen into four quadrants.

To access, assuming that the current App window is not maximised; touch-and-hold the traffic-light icon at top-left of the App window to expose the Fill & Arrange options:

The current App window will move aside, allowing you to open another App; the selected view will open the selected Apps in their own windows - which can be resized laterally, as before, by dragging the divider to left or right.

If the App window is already maximised, swipe downwards from the top-edge of the screen to expose the new menu bar - then use the traffic-light icon as described above.