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 undermine the security of your device and your personal data.
If your Logitech Pencil is not working properly after updating your iPad, you'll need to seek assistance from Logitech Support. As for Apple Pencil, very few reports of issues following update to iPadOS 26.2.1 have appeared here; my own Apple Pencil is working, as expected, without issue.