Windows 10 Mobile security updates will end on December 10th, 2019
Microsoft has announced that all support for Windows 10 Mobile will end on December 10th, 2019.
Following the discontinuation of Windows Phone 7.5 and 8 last year, the end of support for Windows 10 Mobile has been expected for some time. Feature updates for the mobile operating system have been discontinued for some time, which Microsoft itself admitted back in October of 2017.
Device backups containing settings and app data can still be made until March 10th, 2020. After that point, users will have to migrate their settings and miscellaneous data by manual data entry where they can; otherwise, they'll have to start over. Microsoft listed the following steps to make a backup before then:
Settings -> Update & security -> Backup -> More Options, and then tap "Back up now"