Andorid O has brought with it a number of new features, although, one which has now been noted as removed (or at least present in a reduced capacity) is the battery tab within settings. Previously (on Android Nougat), users were able to identify and view more granular battery information and statistics. So for instance, you could click through to settings, then to battery, then click on any of the system-level settings (like Bluetooth, for instance) and view more details on the level of battery consumption based on just that micro aspect, in this case, Bluetooth.
This is what has been removed (or reduced) in the latest developer preview of Android O as while you can still see an overview of how the micro system aspects are impacting on battery life, there does not seem to be any option to view more detailed information. In short, the setting level tabs are no longer clickable tabs with no option to delve further into the level of battery consumption for any specific system aspect. Although, it is understood that battery life in general has been greatly improved with the latest version of Android, by way of the operating system limiting background activity.
While it is expected that a new version of Android would bring more than it does take away, this is not the first feature to be noted absent from this latest version and interestingly, not the first battery-related one. As it was noted yesterday that the battery icon in Android O no longer allows the displayed percentage to be included within the actual battery icon. So it does seem that Google is certainly rethinking how the battery is displayed and the level of battery information that is available with Android O. Of course, with this being a developer preview, and especially with this being the first developer preview, none of these changes are guaranteed. There is always the chance that Android O's added features will be removed in the final product and by the same token, Android O's removed features, reintroduced at a later time. So while the battery settings do seem to be somewhat toned down in the first developer preview, as with everything Android O at the moment, it is subject to change.