With a recent beta (version 2.18.69) update to WhatsApp for Android, developers have extended the limit to delete messages for everyone from the previous limit of 8 minutes and 32 seconds to 68 minutes and 16 seconds. This change was also applied to recent WhatsApp for iOS and Windows Phone updates. As per findings by WABetaInfo in a report, it now appears that WhatsApp has silently gone on to implement a 'Block revoke request' feature in order to stop users from exploiting the 'Delete for Everyone' feature through modded versions of the app.
Since WhatsApp introduced the 'Delete for Everyone' feature back in November 2017, the recipient can understand that a message has been deleted as there is a prompt in its place with the “This message has been deleted” tag. Using this notification system, many users, thanks to modded and unauthorised WhatsApp for Android versions, were able to delete any message for everyone with messages dating back to 3 years.
This is how the safeguard feature works. When the user deletes a message for everyone, WhatsApp sends a new message to the recipient. This message has the same ID of the message that the user wants to revoke. WhatsApp then finds a message in the database having the same ID of the received revoke request and checks the date related information of the message saved in the database.
The algorithm has been improved to perform the delete operation only if the current date is less than 24 hours of the date of the saved message. This ensures that nobody can tamper with the feature to delete messages for everyone.
WhatsApp has decided to stick to 24 hours as the reference time period here to ensure that there is ample time for the recipient or recipients in case of a group chat, to receive the revoke request.