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. This control similarly cannot be disabled or manually hidden.
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
Whilst it is not possible to disable multitasking features of iPadOS15, Apple do invite constructive feedback and feature requests via their product feedback pages:
Feedback - iPad - Apple
If you have particularly strong feelings about changes made within the new OS-release, or have suggestions for improvements, the feedback page is perhaps the most appropriate channel through which to express your views. Whilst you are unlikely to receive a reply, it is understood that all comments are logged and forwarded to the most appropriate team.