Through Android's progression and maturation over the years, features have come and gone as not everything ends up working out. When this happens Google usually ends up removing them from the OS entirely, because why continue to keep and offer something if it isn't working or isn't a popular functionality? Sometimes, though, Google removes features that are used rather often, like the do not disturb mode function that allows users to set their notification alerts to be disabled until their next alarm. This was introduced back with Android Lollipop, but was later removed for Android 6.0 Marshmallow users. Just a few weeks ago it was brought back with the arrival of Android 6.0.1 for those devices which have been updated to it.
In a weird twist of events it seems that more than a handful of users are reporting in forum threads and on Reddit that the do not disturb setting until next alarm is disappearing from their notification alert options, with more than one user reporting that they had used the feature with Android 6.0.1 as recently as yesterday evening, only to wake up this morning and find that the option is no longer visible, seemingly having disappeared from the device completely.
While this may not be an issue for everyone, many users rely on the do not disturb until next alarm to keep their phone on silent for a portion of time until the next alarm goes off. Many users enable this function before going to sleep so that notifications aren't coming through relentlessly throughout the night. Of course, the "none" option is still there, but this requires users to remember to turn the notifications back on, and this is an easy thing to forget. According to the Reddit posts, users with the four most recent Nexus smartphones have all reported issues with this feature being no longer available to them sometime over the course of the weekend. This includes users on the Nexus 5 and Nexus 6, as well the Nexus 5X and Nexus 6P, so it doesn't seem to be isolated to just one device type. Other users are reporting that it may have something to do with the date on the device and that turning back the date to before December 25th seems to cause the setting to show up again. It should be noted that modifying the date is not a temporary fix as it would likely cause other issues, and it seems this is just something users will have to wait for Google to send out a patch for.