Double Tap to Wake/Sleep is a feature that is easy to get used to and difficult to live without once you do get accustomed to it. Most smartphone users - Android or otherwise - however, continue to live without it, blissfully unware of what exactly they're missing out on. Double Tap to Wake and Sleep of course, was one of the many innovative features that was first seen on a CyanogenMod release a few years back, and has since been adapted by a number of Android vendors around the world. On a mainstream smartphone, the feature was first seen on LG's 2013 flagship smartphone, the G2, and is currently natively available on just about the entire current-generation LG smartphone lineup, starting with its flagship G4, right down to its entry-level devices.
The feature of course, is also available on devices from vendors like OnePlus, whose first generation flagship, the OnePlus One came with the feature out of the box, as it had Cyanogen OS pre-installed on it. Taiwanese OEM Asus's ZenFone 2 range of smartphones also support the feature out of the box. While plenty of apps available on the Google Play Store promise to help just about any device get the feature, absolutely all of them require root access, meaning most regular users will continue to have to live without it in the foreseeable future as Android is yet to natively support Double Tap to Wake/Sleep as of 6.0 Marshmallow.
Talking about Google's reluctance to include the few lines of code required to incorporate the feature into Android natively, a new post on XDA by recognized developer flar2, known for his ElementalX kernel, seems to suggest that the Nexus 6P apparently already has support for the Double Tap to Wake feature, even though it is disabled by default on Google's flagship smartphone. Not only that, there is no way to enable it through the GUI in the phone. He says that he stumbled upon the code for built-in double tap gesture while he was going through the source code. While he has promised to add the piece of dormant code to his kernel, it will still only be restricted to users with root access, meaning the large majority of Nexus 6P users will continue to make do without the feature till Google decides to add the code to AOSP. There is however, still no word on whether there's hidden code to support the Double Tap to Sleep gesture as well on the Nexus 6P.