If I have to accept new terms, where I can read them up?
For the case of Whatsapp, I can say today, that they simply made it not easy accessible to read up their terms of service. They had as stated in OP not updated the terms of service for more than 4 years while not making them publicly available. The linked side got updated once last year. But still Whatsapp expects you to accept their "recent changes in terms of service", leaving you without any clue where they are published.

Lets see if they will maintain it now or that source will be outdated soonish again.