Honest question: Can I use the "last seen" time as a rough proxy for "they have read your message"? I've been wondering this for a while.

At least on Android, my 'last seen' counter doesn't update until WhatApp is brought to the foreground on my chat partner's phone, which is normally a good sign that they have read my message. If they just see the notification but don't open the app my 'last seen' counter does not update