According to WABetaInfo‘s newest post, it seems like WhatsApp is working on its newest feature called “Keep message” which will be available on Android, iOS, and Desktop. The feature is still being developed and may arrive in future updates.
When users share an important message in a disappearing chat and don’t want to lose it, they can use this feature to save it by preventing it from expiring. In simple words, you can prevent some disappearing messages from disappearing in a future WhatsApp version.