Stamina Mode removed in Marshmallow
Sony announced in their beta rollout for Marshmallow
The Marshmallow version of Android has its own smart solution for managing power consumption (Doze and App Standby). These functions are integrated directly into Android 6.0, which made Stamina mode redundant. In other words, Stamina mode was removed from M.
However, Stamina Mode is considered better than Doze mode ( for details see earlier link) :
Doze mode requires your phone to be geographically stationary (ie. sitting on a desk and not in your pocket).
Doze mode can only be interrupted by high priority GCM messages. Your favorite app’s developer didn’t set a notification as high priority? Too bad, you have to wait until the next maintanence window to be notified.
Doze mode’s white-list does not truly exempt an app from all restrictions.
Doze mode cannot be disabled. Having issues with notifications on Android 6.0? Too bad.
Stamina and Ultra Stamina Mode Return
Stamina Mode users resentment at this perhaps forced Sony to reconsider and Stamina Mode and Ultra Stamina Mode have been announced as below:
Not as good as original Modes
But the Stamina Mode is said to be a watered down version and is more akin to stock Android Power Saving mode ( mainly because whitelisting of apps isn't possible):
Presumably, Sony felt immense pressure from customers who wanted STAMINA back, but also did not want to break Google’s terms and conditions for using Android 6.0 Marshmallow. Quite where Sony will go now with their device software remains to be seen, but as it stands, the new STAMINA mode is simply not as effective or as usable as the old way.
As of now I haven't come across any mention of Stamina or Ultra Stamina Mode in Android 7