Microsoft has now posted an FAQ so customers can get more information about a recent decision.
According to 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.
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.