Meta is developing a method to prevent messages from disappearing on WhatsApp. Users only need to add a message to their bookmarks to avoid it disappearing in the future. If the user chooses to remove (unkeep) the message from their bookmarks, it will permanently disappear.
This feature is currently being tested on WhatsApp Android version 2.23.8.3 and iOS version 23.7.0.72, and is only visible to limited beta users. It is expected that testing will be expanded in the coming weeks.