I don’t ever want a split screen. How do I turn this function OFF?

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 (or not using) 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

Being aware that now (running iPadOS 15 or later) the only option to access/control multi-tasking is the “three-dots” button at the top center edge of the screen (as per these instructions:--> Use multitasking on your iPad - Apple Support), you can submit your comments/requests to Apple directly, using this link:--> Apple – Product Feedback.

The more requests are made, the more likely it is that Apple will change the current settings for this widely used feature on iPads.

Regards

Giulio

You are most welcome