WhatsApp owned by Facebook-owned messaging app will withdraw support for a number of dated smartphones by the end of this year. The company has announced that the app will not be usable on Windows Phones after
December 31, 2019.
Notably, the app will also stop working on some older versions of Android and iOS starting early next year. WhatsApp has updated its FAQ page to make the announcement.
Once the support is withdrawn, you won't be able to create new accounts or re-verify existing accounts on a Windows phone starting January 2020.
The primary reason for stopping support for older operating systems is because WhatsApp will no longer actively develop for these operating systems, and hence some features may stop working abruptly. WhatsApp has also highlighted that it'll offer support for Android versions 2.3.7 and older as well as iOS 8 and older until February 1, 2020.
That means, if your device is running on an outdated OS, you won't be able to use WhatsApp from February 2, 2020.
Hence, you should upgrade your device's software or get a new phone which runs the newer version.