According to WhatsApp beta news blog WABetaInfo, WhatsApp is beta testing a feature that hides a user’s ‘last seen’ status from specific contacts.
‘Last seen’ on WhatsApp informs other users when someone last opened the app, which might indicate whether they checked it without responding to a message. You can now hide your ‘last seen’ from everyone, but you won’t be able to see anyone else’s. You can also limit who sees ‘last seen’ to just your contacts, but you can’t pick and choose who sees it.
The new functionality will add “My Contacts Except…” to this list, allowing you to restrict who can see your “last seen.” It’s now in beta for iOS and Android, and should be available to all WhatsApp users soon.