The developers over at WhatsApp are finally close to delivering chat history migration between Android and iOS devices. That was revealed by the source code found of WhatsApp 22.2.74 beta for iOS.
It appears that it won't be a straightforward process, though. The migration process requires both devices to have the app installed and the process of migration will likely work using a cable or a private Wi-Fi connection. In comparison, other messaging apps store and sync user data using Google Drive, iCloud or their own cloud service.
As of now, the only official way to sync your WhatsApp data between an iOS and Android phone is to own a Samsung-made device. The migration works one way - from an iPhone to a Samsung via cable and using Samsung's SmartSwitch app.