Sony has long offered a feature on its phones called Stamina Mode. When activated, unauthorized apps can't run in the background or run sync jobs. The problem, however, is Stamina Mode interferes with Android 6.0 Doze Mode. Sony took it out of early Marshmallow builds, but the company has now confirmed it's coming back.
According to Sony, Stamina Mode will return to the Xperia Beta Program soon. It doesn't have a firm schedule, but Sony has apparently found a way for Doze and Stamina Mode to coexist. Google requires that OEMs not interfere with the way Doze mode operates, but many Sony fans prefer Stamina Mode as there are user-facing options and it works all the time, not just when the device is stationary. Although, Stamina Mode in Marshmallow might not work the same way.
The Xperia Beta Program has been running for a few months, and will probably last a few months more. It's only available on recent Z series devices, and limited to a few European countries. Everyone else will have to wait and see what Sony's doing with Stamina Mode.
XperiaBlog reports that Stamina Mode is actually back (pause for fireworks) for real on NTT Docomo's Xperia Z4, Z5, Z5 Compact, and Z5 Premium devices that have received an OTA update to Marshmallow from the Japanese carrier. It's now a matter of time before it shows up on other Marshmallow builds across the world.