Apple - 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
If you have a Mac computer you can download and install Coconut Battery on your Mac. Then connect your iPad to your Mac with a USB-lightning cable and run Coconut Battery. Click on "ios device" on the top of the Coconut battery display on your mac and this will tell you much about the battery in your ios device.
If you don't have a Mac go to the App Store and search on battery. There are many apps that claim to give you info about your battery. These provide different levels of information so you'll have to search for one that provides you with what you want. Many are free so there is plenty to try. I can't really recommend one since I use Coconut.