"For the last several weeks we've displayed a notification in WhatsApp providing more information about the update. After giving everyone time to review, we're continuing to remind those who haven't had the chance to do so to review and accept. After a period of several weeks, the reminder people receive will eventually become persistent," the WhatsApp FAQ page reads.
WhatsApp will not stop all the functions at once. It will send repeated reminders to users failing which the users will be barred from using some features. The first and foremost thing that the user will not be able to access is the WhatsApp chat list. That said, WhatsApp will still allow users to answer incoming phone and video calls. And if you have enabled the notifications, you can tap on them to read or respond to a message or call back a missed phone or video call.
If you delete your account, your message history will be deleted, and you will be removed all of your WhatsApp groups and all your backup will be deleted too.