I'm using Nokia Lumia 520 version 8227. When I log into WhatsApp, there's a caution about my phone being unsupported with a message that WhatsApp for Windows Phone expired on 31/12/2017.
Can I still use WhatsApp on Lumia 520?
According to WhatsApp FAQ, support for Windows Phone 8 has been discontinued on December 31, 2017.
WhatsApp will no longer be supported on Windows Phone 8.0 after December 31, 2017. This means that if you are currently using WhatsApp on a Windows Phone 8.0, you will be able to continue to use WhatsApp on that phone before this date.
Solution: Update your phone to Windows Phone 8.1. According to this softpedia article Lumia 520 did recieve an update to Windows Phone 8.1.