As reported by Thurrott, Microsoft have updated their Windows 10 Mobile FAQ to state that support for the platform will end on December 10th this year. The official advice? Switch to Android or iOS.
As per the FAQ:
As of December 10, 2019, Windows 10 Mobile users are no longer eligible to receive new security updates, non-security hotfixes, free assisted support options, or online technical content updates from Microsoft for free. Third parties or paid support programs may provide ongoing support, but it is important to recognize that Microsoft support will not publicly provide updates or patches for Windows 10 Mobile.While once again Windows Mobile isn't completely, officially, totally dead, the cessation of OS support is a pretty significant death knell. It remains to be seen how this will affect any remaining Windows 10 or UWP games currently available to Windows Phone users, but we can guess that support from third party will be fairly slim if there's no first-party support to back it up.
Only device models that are eligible for Windows 10 Mobile, version 1709 are supported through the end date. For Lumia 640 and 640 XL phone models, Window 10 Mobile version 1703 was the last supported OS version and will reach end of support on June 11th, 2019.