Special utilities such as the WhatCrypt Tools should be able to help you with a conversion / recovery of various WhatsApp message backups (for example .crypt12 introduced in June/July 2016) and save the chat history as .zip file that contains the chats in a HTML format that you can view using any web browser.
Once you have the HTML file, it's just a matter of exporting and saving it as TXT file . But remember that crypt12 to txt conversion will also remove any formatting and you will just get plain text.
Open CRYPT12 file Open TXT fileOnline service for WhatsApp databases