Rolling-back to an earlier version of iOS/iPadOS is not possible. All System Updates are digitally signed by Apple - and can only be installed whilst the digital signatures remain valid. After release of a new iOS/iPadOS version, the immediately preceding version continues to be digitally signed by Apple - however, usually within a few days, Apple will revoke the digital certificate with which the earlier version is signed.
Perhaps consider that iOS/iPadOS updates include both functional and essential security updates. Restoring an earlier version of the OS would expose your device, privacy and data to emerging and known security threats and vulnerabilities.
If you do not wish your iPad to receive automatic system software updates, you must ensure that the feature is disabled in iPad settings:
Settings > General > Software Update > Automatic Updates > [iPadOS Updates]
- Automatically Install - set to OFF
- Automatically Download - set to OFF
Settings > General > Software Update > Automatic Updates > [System Files] Automatically Install - set to OFF