There was a recent thread on iOS Whatsapp. The short of it is: third-party costs some bucks, but is the path of least resistance: Impossible to transfer WhatsApp data from iOS to /e/OS - #9 by Hoogan
If you have a regular Android phone around (or before switching to /e/OS) you could probably do iOS → Stock Android (with Android Switch) → backup WhatsApp archive, use Android export tooling for iMessage and else → /e/OS Android.
The forum has some “ios” migration threads (this one), but not much bundled solutions, frankly the iOS section at Migrate to /e/OS is slim.