Apple has made a somewhat strange change with iOS 15.6. As of this version, when we consult the location of our AirTags from the Find application, we can no longer see your battery level in the interface. If you have not yet updated to iOS 15.6 you can see the battery symbol with the rest of the AirTag information.
If it cannot be measured well, we better not measure it
We could attribute it to lapse from the developers, but it seems that the change has been intentional: references to that battery indicator have disappeared from the documentation from Apple support. And furthermore, that same indicator has also disappeared from the Find app from iPadOS 15.6 and macOS Monterey.
Apple hasn’t detailed the exact reason for this and so we’ll never know for sure, but there’s some speculation that that indicator used to show an incorrect battery level (at least in some cases). Perhaps Apple has preferred to get rid of the problem seeing that the measurements of that battery are too imprecise. Let us remember that AirTags do not have a battery, but rather a button cell with which must be more inaccurate measure the load.
This does not affect the alert that will appear when the battery level is low, of course. We will always know the best time to change the battery in the AirTag. In theory they last a year, but in my case I’ve been with it for almost 15 months and I still don’t have any low battery warning. It is not bad that a product has more autonomy than announced for a change.