Well, WhatsApp will not delete your account immediately as it has its terms for its users to read it at their pace but eventually, it will restrict your usage. Here's what the Company's FAQs say, "To give you enough time to review changes at your own pace and convenience, we've extended the effective date to May 15th."
With time, if you do not accept the terms and policy, you will only receive calls and notifications and not see or read any text messages.
What happens if the account gets deleted? The company said, "It is something we cannot reverse...," which means it is permanent. The users will not even have any history of their messages.
We all are in some groups or the other in WhatsApp. Now, if we don't accept the policy, we will be removed from all groups and won't be able to access them.
Even if the company is giving time till May 15 to take backup of the chats, after May 15, users will lose the entire WhatsApp backup. To save your conversations, you can export them before the deadline as per the company.
The company said, "You can still accept the updates after May 15th. Our policy related to inactive users will apply."