Microsoft today will deliver the final security update for Windows 10's first feature upgrade, the version released in November 2015.
Windows 10 1511 -- Microsoft labels its feature upgrades in a yymm format -- will receive its last security patches, then fall off Microsoft's support list. The company had announced 1511's retirement several times in the past, notably in a support lifecycle fact sheet on Microsoft's website.
Today is also October's "Patch Tuesday," the month's release of security updates for Microsoft's products. The company has tied the end of support for Windows 10's feature upgrades to Patch Tuesdays. The next in line after 1511, last year's 1607 -- aka "Anniversary Update" -- has been tentatively scheduled to drop out of support in March 2018. If true, Microsoft will undoubtedly call it quits on March 13, 2018, another Patch Tuesday.