How do I disable split screen?

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

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

Using iPadOS15, multitasking or split screen cannot be turned off in settings any longer.

Now you need to use the 3dots button at the center top of the screen.

All details are provided in the support article that led you here.

Regards

Giulio