Will support OS 4.0.3, and iOS 9 and newer
WhatsApp will stop working on certain older versions of Android and iOS starting January 1.
Their updated FAQ section says that the software will be supported on Android running OS 4.0.3 and newer, iPhone running iOS 9 and newer, and select phones running KaiOS 2.5.1 newer, including JioPhone and JioPhone 2.
This means that i Phone 5 and newer will support the software. Old HTC, Motorola, LG and Samsung Galaxy S2 phones will not support the updated WhatsApp.
If you have an Android, you can check what version you have by going to Settings>About Phone. For an i Phone, go to Settings>General and Information>Software.