I don’t think Apple will ever include this. Not only because it’s been present since iOS 11.3 for iPhones and they’ve never made it available for iPads, but also because of how the entirety of Apple’s iPad battery health assessment for replacements goes.
Apple does not replace the battery on iPads. Apple replaces the entire iPad. To do so, you’re required to perform an Apple-assessed battery test which never matches available commercial tools to assess health. Apple’s number on iPads is always at least 9-10% higher than Coconut, whereas it matches the iPhone’s Settings-Battery-Battery Health number. Apple has no interest in changing that. Nobody knows exactly what Apple’s formula is for the number.
I will give an example: my 9.7-inch iPad Pro has 84% health on Coconut. I asked Apple, and they said it is at 93% health. Everyone reports similar results.