Check battery health on iPad like on iPhone
Battery Health is currently not available for the iPad.
According to Apple Support,

For iPhone 6 and later, iOS 11.3 and later add new features to show battery health and recommend if a battery needs to be replaced.

Nowhere on the page does it mention iPad or iPadOS, so unfortunately, at this time, it's limited to iPhones running iOS 11.3 and later.
You may want to submit a feature request with Apple at https://www.apple.com/feedback/ipad.html