Last November, WhatsApp introduced messages that disappear after 7 days. A couple of months ago it also started work on "view once" messages, that are gone forever after you viewed them once, as the name handily implies. Now it turns out that WhatsApp is also going to add another option for message expiry, and that's 90 days. So whenever all of these features make it out of beta, you'll be presented with a menu similar to the one you can see in the screenshot below, where you'll be able to pick between "Off" (so messages never disappear), or choose to have your chats disappear after 24...
Source: https://www.gsmarena.com/whatsapp_is_working_on_messages_disappearing_after_90_days-news-50575.php
