Honestly, I saw some dialog and clicked Accept, despite not reading it. I don't use WhatsApp for anything critical. I use iMessage for that and I have a lot of trust in Apple to do the right thing.
Yeah I am aware of that one. They postponed the update, so your link is not actually relevant in terms of operational iOS phones right now. Let's see what happens in the next months.
iMessage could be great except it falls back to SMS when the other phone is Android or there is no data network available. Also you lose the privacy once your messages are backed up to iCloud.
If they are not on iMessage, you will see that before even sending a message to them. At that point I'll go and see if they have some other means of Instant Messaging like WhatsApp or Telegram.