If you're not aware, System UI Tuner is a hidden setting in Android that allows some degree of interface customization. It doesn't have as many abilities as custom ROMs, mind you, but still contains some very useful settings. One feature in System UI Tuner allows you to add the battery percentage to the status bar inside the battery icon.

Unfortunately, in the Android O Developer Preview, placing the percentage inside the icon is no longer an option. This will no doubt annoy some customization enthusiasts, but the percentage can still be displayed alongside the battery icon.

 

Left: Android 7.1.1 Nougat; Right: Android O Dev Preview

As with everything in the Android O Developer Preview, this is all subject to change. The final release is several months away, so there's plenty of time for features to be added or removed.