Reset Ipad from an old email that doesnt exist anymore from ex employee

If the company is the original owner, have a designated company employee contact Apple. Apple can remove the ID  if provided the required paperwork to include the original purchase receipt with the serial number on it. 

Request additional support for unlocking your device.    -

You used to be able to take the device, the original purchase receipt with the serial number, and a photo ID to the nearest Apple Store (if it has reopened) and have them remove the ID. This may have changed, so call the store before going to make sure they can help.

Genius Bar Reservation and Apple Support Options - Apple.   -

you need their sign in credentials.Where you using Mobile Device Management? Get started using Apple Business Manager or Apple School Manager with Mobile Device Management - Apple Support