That WhatsApp for Windows Phone is the least supported version compared to Android and iOS is no secret. However, now WhatsApp was pulled from the Windows Phone Store and cannot be downloaded currently.
The WhatApp Team gave following statement regarding the issue:
Unfortunately due to technical issues, we have chosen to un-publish WhatsApp Messenger on the Windows Phone platform. We are working closely with Microsoft to resolve the issues and hope to return to the store shortly. We apologize to our users for the temporary inconvenience.
Let’s hope that WhatsApp will be very soon available again and moreover receive a proper update.
Source: WP Central