In a statement to Crackberry, Once the date passes Dec 31, 2017, the app will go into “deprecation” mode. It will still continue to function for a short grace period, but some behaviors will change. Most importantly, you will no longer be able to re-register your account with the app. That means if you delete and reinstall it (or install it on a different phone), it will no longer allow you to verify your phone number and complete the setup process.
After the grace period, which is approximately two weeks after the published date, the app will go into “expired” mode. At this point, it will stop functioning as a usable WhatsApp client, and thus will no longer connect to our servers. (Around this same time, our servers will stop allowing the client to connect as well.) However, once it is expired, the BlackBerry 10 client app may still allow you to access your existing message content on your phone. This behavior should be similar to what was seen by everyone who forgot to update their app the last time our published expiration date had passed.
Source: {infocity blog}