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 avoiding) 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
As your iPad Pro is capable of update to iPadOS 16.1.1, why not consider updating to this version? If you enable Stage Manager, you’ll have other options to control multitasking - which may better suit your workflow.
Turn Stage Manager on or off on your iPad - Apple Support
https://www.imore.com/how-use-stage-manager-ipad