Microsoft is pretty diligent about posting end-of-life dates for its products, making it clear how long they will be supported until they aren't anymore. For Windows 10 Mobile version 1709, the final version of the OS, that date was December 10, 2019. On that day, Windows 10 Mobile was finally dead.
Until it was revived. A few short days later, Microsoft updated a support document to say that Windows 10 Mobile will actually no longer be supported after January 14, 2020. There was no explanation posted, and we even reached out to the Redmond firm and couldn't get a proper answer.
At the time and to this day, extending support for the OS seems like a very unlikely thing to do. Very few people are still using it. Even the die-hards hanging onto their Lumia 930 units couldn't get anything beyond Windows 10 Mobile version 1607. Version 1709 was exclusive to phones that actually shipped with Windows 10 Mobile.
But as of today, the failed OS is finally dead. Well, it should be dead. After what happened last month, frankly anything is possible.