If your iPhone runs an iOS version older than 15.1, you will no longer be able to run WhatsApp on that device as of May 2025.
On May 5, WhatsApp will remove support for the iPhone 5s, iPhone 6, and iPhone 6 Plus, which have not received a software update after iOS 15.1.
Fortunately, these are very old iPhone models from 2013 and 2014. If you are still using such an oldtimer, it might be a good time to consider an upgrade.
You will not lose your WhatsApp chat history: you can back it up and restore it from iCloud.