Unable to update WhatsApp on iPhone despite available storage

WhatsApp is not an Apple product. Contact the app developer here --> App Support or WhatsApp FAQ  Or Restoring your chat history - WhatsApp FAQ or Contact WhatsApp

WhatsApp Business is not an Apple product. Contact the app developer here --> Developer Website  or App Support  or WhatsApp business products - WhatsApp FAQ

• iOS does not block third-party app updates unless there is a security concern (such as a known vulnerability in an app). However, Apple may remove an app from the App Store if it violates guidelines, preventing updates.
• Apps that are not updated for a long time do not get "locked," but they might stop functioning properly if they rely on newer iOS features or APIs.
• Managed Device (MDM) Profiles can restrict app updates, installations, or removals on corporate/enterprise-managed devices. If an MDM profile is installed, IT administrators can enforce app version control and prevent updates.

Your tag line says that you have an iPhone 8 Plus running iOS 14. Whatsapp Messenger requires iOS 15.1 or higher.

You can update an iPhone 8 Plus to the latest release of iOS 16, so if you are still on iOS 14, update to iOS 16 – and then try updating WhatsApp.