Can’t find out how to turn off split screen or multitasking setting

Presumably your iPad is running iPadOS15.

Unlike iPadOS14 and earlier versions, multi-tasking cannot now be disabled from settings. Multi-tasking has received a significant overhaul, both simplifying its use and introducing new controls.

Key to accessing and controlling multi-tasking is the small “three-dot” button that is found at the top-centre edge of the screen.

More information about using multi-tasking can be found in this support page:

Use multitasking on your iPad - Apple Support

The iPad User Guide is also a rich source of information about using iPad and its many features. 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

To reiterate, in iPadOS15 you cannot disable multitasking - or “cripple” split or slide over views.

Whist not being the solution that you seek, the very best available solution is to understand how multitasking is accessed and controlled. Armed with this knowledge - and with a little practice - you’ll find the enhanced OS to be remarkably flexible and productive. The previous iteration, iPadOS14, seems remarkably archaic and limited.

All that said, Apple do invite submission of constructive comments and feature requests via their Product Feedback portal. For iPad and iPadOS:

Feedback - iPad - Apple