The iOS shows a phone number input if you try to initiate a chat and have contacts permissions disabled. I'm not sure whether that's because it's to comply with app store regulations, or the product owners on both sides prioritizing different things. On both platforms you can still initiate chats without contacts access by using the wa.me domain, eg. wa.me/12125551234 for +1 (212) 555 1234.

That's not true for me on iOS. I denied it access to my contacts, and I can still start a new conversation by entering a recipient's phone number.