According to a support FAQ on the company’s website, WhatsApp is ending support for a number of older operating systems (OS), including Windows Phone.
The post recommends using Android 4.0.3 or later and iOS 9 or later. Some phones running KaiOS are supported, with 2.5.1 being the oldest version recommended. Notably, WhatsApp on Android 2.3.7 and older cannot create or reverify accounts, although it can still be used until February 1, 2020. The same applies to iOS 8 and older.
Windows Phone users will not be able to use WhatsApp at all, effective December 31, 2019, as the company is completely ending support for the OS.
As the post points out, there is no option to transfer chat history from one platform to another. Users who want to keep their history for reference, however, can export it as an email attachment. Any Windows Phone users, in particular, should take advantage of that feature in the little time they have left.