Your iPad still being supervised indicates that a management profile is still present - and that the device is still centrally managed by an Enterprise or Educational establishment. Devices that are supervised/configuration locked by an MDM platform can usually only be released from central management by the platform that manages the device. Be aware that Apple will not assist with removal of management profiles.
Unless you are lucky - and have the option to delete the profile from iPad Settings - you’ll need to seek assistance directly from the System Administrator who manages the device.
Install or remove configuration profiles on iPad - Apple Support