Permanently disable split screen on iPad?

Presumably your iPad is running iPadOS15.

Unlike iPadOS14 and earlier versions, multi-tasking cannot 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 avoiding) multi-tasking is the small “three-dot” (ellipsis) 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

Apple do invite constructive feedback, comments and feature requests via their product feedback pages. For iPad and iPadOS:

Feedback - iPad - Apple

You are encouraged to make use of this facility in expressing any views concerning issues, bugs, or functionality changes of which you are unhappy.

There is no way to permanently disable multi-tasking on iPad.

Feel free to provide feedback to Apple via this link, but don't expect this to change as it's something the masses begged Apple for for a long time --> Feedback - iPad - Apple