The server shutdown in 2017 was the final nail in the coffin. No amount of hacking, modding, or firmware patching can resurrect WhatsApp on the J20i because the central service no longer accepts its login token.

on it today is highly restricted as official support has long ended. WhatsApp Support Status Official Support:

The old J20i couldn't speak the language of the modern internet. The encryption, the security certificates, the data format—it was all too new for this old soldier.