It is now possible to transfer WhatsApp chat history from iOS to Android. However, this is still not a straightforward process as it involves connecting the phones physically and initiating the transfer. Currently, this feature is only supported on Samsung Android phones with support for other Android phones coming later.
https://www.notebookcheck.net/WhatsApp-now-allows-chat-history-transfer-from-iOS-to-Android-but-there-s-a-catch.558406.0.html