Sony’s Stamina mode was a feature in older software builds that served as a battery saving mode. It shut off many different processes and features to preserve your precious battery life. Unfortunately, with Google including Doze mode in Marshmallow, Sony had to remove the feature so it wouldn’t interfere.
The company promised that the feature would return, and it finally has, though only in the beta Marshmallow 6.0.1 builds for the Xperia Z2, Z3, and Z3 Compact. It’s also very limited now to avoid interfering with Doze mode, but it’s back and it’s working with the limitations set by Google’s new software.
This also means that Stamina mode will come to official builds eventually, so keep an eye out for it in your next software update!