Split-view and slide-over App views are still available within iPadOS 26 - although they are accessed a little differently from earlier versions of iPadOS.
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. This short video may help you to understand how to use the multitasking system:
https://cdsassets.apple.com/vid/live/hls/7WUAS350/videos/93ad67d2_1759269510.m3u8
You would also likely benefit from reviewing 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:
Multitask on iPad with iPadOS 26 - Apple Support
Work with multiple windows at once on iPad - Apple Support
OS - iPadOS 26 - Apple
Apple invite submission of constructive comments and feature requests via its Product Feedback portal. If you would like to see a specific feature or change to functionality in a future version of iPadOS, this would be the most appropriate channel through which to make a request:
Feedback - iPad - Apple