How to bring back my battery health

You may try recalibrating it.

Recalibration in iOS 14.5 and later

iOS 14.5 includes an update where the battery health reporting system will recalibrate maximum battery capacity* and peak performance capability on iPhone 11, iPhone 11 Pro, and iPhone 11 Pro Max to address inaccurate estimates of battery health reporting for some users.

About recalibration of battery health reporting in iOS 14.5 ...