Do you have other iPad/iPhone (or Mac) devices - upon which you have enabled Advanced Data Protection and/or Account Recovery features?
Settings > [Your Name / AppleID] > iCloud > Advanced Data Protection
If you have enabled these features on any of your devices that are running iOS/iPadOS 16.2 or later, you will not be able to sign-in with your Apple credentials on any devices that are not running iOS/iPadOS 15.x or later.
If you disable iCloud Advanced Protection on all your devices, you will be able to sign-in on your older iPad.
Should you wish to continue use of iCloud Advanced Data Protection, you will need to first disable Find My on the older device - then fully sign-out from iCloud - before enabling Advanced Data Protection for the newer devices. You can then continue use of the older device(s) using a different AppleID.
How to turn on Advanced Data Protection for iCloud - Apple Support