Since Marshmallow, Google has used the hidden System UI Tuner menu to hide useful customisation options in the OS. The first Developer Preview of Android O is no different and hides some interesting customisation options under System UI tuner, with one of them being the ability to customise the lock screen shortcuts.
The feature is pretty simple and essentially lets you set custom shortcuts for the right and left shortcuts on the lock screen. You are not limited to setting the shortcut to open a particular app. If you want, you can set a shortcut to jump directly to a particular section of the Settings menu, open a new Chrome tab, do a quick Google search, launch the selfie camera app, and more.This feature has long been available on custom ROMs and Google is finally getting around to natively adding it to Android with O.
Since the feature is buried under the System UI tuner, it is possible that Google will remove it from the final version of the OS. The chances of that happening seem pretty unlikely to me, though.
If you are running the Android O developer preview on your Pixel or Nexus device, you can bring up the hidden System UI tuner menu by long pressing the cog Settings icon in the Quick Settings menu for a few seconds. When you leave the button, you should get a toast notification saying that the System UI tuner menu has been enabled. Then, head over to Settings -> System -> System UI Tuner -> Lock screen to customise the lock screen shortcut.
Since Lock screen shortcuts are hidden under System UI tuner menu, it is possible that they will either graduate to a full blown feature by the time the final version of the OS is released. Or if Google still thinks that the feature is half baked, it can remove the feature from the hidden System UI tuner menu altogether as well.
Do you want Google to offer the option to customise the lock screen shortcut in Android O?