Microsoft has shipped Patch Tuesday updates for all supported
versions of Windows, including Windows 10 Mobile version 1709.
The September 2019 cumulative update for Windows 10 Mobile
comes without any noticable improvements or fixes.

The update advances the system to Windows 10 Mobile Build
15254.587 (up from 15254.582) and it comes with the following
security-related changes:

• Security update to Windows Kernel, Windows Media, Windows
  Storage and Filesystems, and Windows App Platform and
  Frameworks.

Windows 10 Mobile Build 15254.587 can be downloaded by checking
for updates in the Settings > Update & Security > Phone
update. According to support documentation, Windows 10 Mobile
will stop receiving the cumulative updates in 2020. The last
update could be shipped on December 10, 2019.

The company will stop releasing security and software updates
in 2020, and it will also end technical support for the devices
on December 10.

Microsoft says Windows Phone users can only create backups for
settings and supported apps until March 10, 2020. In 2020, some
services will stop working but you will be able to upload and
restore photos from backups for some months or one year after
Microsoft ends support for the OS in December.

You can keep using Windows Phones after December 10th or you
can install Windows 10 ARM. The Redmond firm recommends users
to start considering a switch to Android and iOS where
Microsoft services are actively supported.