I think iMessage and whatever Google had at the time were “good enough” here that WhatsApp never caught on? Like most people already had unlimited texting by the time it hit the scene, so It just felt like a scam back in the day and I remember it wanted my phone number to complete a sign-up and I was damned if I was going to give it to them.
Probably my yahoo mail account. I’ve mostly transitioned off it, but a few things still come to it and it predates even my gmail account (which I barely ever use either). Apple ID’s Hide My Email ability has basically rendered all the other ones pointless though since I can just generate a new fake email address whenever I want to register for something fishy. I wish phone numbers worked the same way.