Windows 10 Mobile will receive its last patches and security updates on December 10 this year, as Microsoft winds down the last remaining bit of development of its smartphone platform.
The last major notable to the platform was October 2017, when it was bumped to version 1709. At that point Microsoft ended feature development entirely, shipping only security updates and bug fixes. That's going to come to an end on Patch Tuesday this coming December.
Certain online services will continue to operate beyond that date; device backups for settings and applications will work for three months, to March 10, 2020, and photo uploads and restoring devices from backups will work for 12 months beyond the end of support.