With the new Windows as a service model, each version of Windows 10 is due to end support after between a year and a half and two years. The original version, now called 1507, was slated for an end of life in March, and that was later pushed back to May.
Version 1511, the first feature update for the OS, was sunsetted last month, but today, Microsoft announced that it's extending support for a full six months, until April, 2018. The change applies to those using Windows 10 Education or Enterprise, as those on consumer SKUs should have been upgraded long ago to a newer version.
After 1511 was released in November, 2015, there wasn't a new feature update until the Anniversary Update in August, 2016. Interestingly, today's change means that version 1511 actually has a later end of life date than version 1607, which is still listed as "tentatively March 2018". Presumably, this date will also change at some point.
Microsoft has previously stated that its goal is to actively support the two latest feature updates of Windows 10. Now that 1511 is back in the fold, it's supporting four versions, including 1511, 1607, 1703, and 1709.