A future version of WhatsApp will let users migrate their chats data across platforms. The feature is currently available in a beta version of the app and only allows iOS to Android transfers. There is no word as to when it will be available on the stable version of WhatsApp, though.
https://www.notebookcheck.net/WhatsApp-will-soon-allow-users-to-transfer-chats-between-Android-and-iOS-devices.530988.0.html
If only WhatsApp did this way earlier when Microsoft officially stopped supporting Windows 10 Mobile, then I wouldn't have to start from scratch when I was forced to go back to Android. I still don't like going back to Android after I left it in mid-2016 in favour of Windows Phone.