How to remove split screen on iPad?

Unlike iPadOS14 and earlier versions, multi-tasking (including split-screen and slide-over views) cannot now be disabled from settings. Multi-tasking has received a significant overhaul with iPadOS15/16, both simplifying its use and introducing new controls. 

Key to accessing and controlling (or avoiding) multi-tasking is the small “three-dot” button that is found at the top-centre edge of the visible App windows. 

Information and guidance for using multi-tasking can be found in this support page:

Use multitasking on your iPad - Apple Support

The iPad User Guide is a rich source of frequently overlooked information about your iPad and iPadOS. The Guide is available both online using a web browser (such as Safari), or as an Apple Books download:

iPad User Guide - Apple Support

https://books.apple.com/book/id1567104892

Apple does invite submission of comments and feature requests via its Product Feedback portal. If you would like to see new configuration options in a future version of iPadOS, that perhaps suppress multitasking views, this would be the most appropriate channel through which to make your request:

Feedback - iPad - Apple