Short answer: Yes, but only if you previously downloaded WhatsApp on your Apple ID.
Apple’s policy requires that the app must have been purchased (or downloaded for free) at least once using your Apple ID on any device. If you have a brand new Apple ID that has never downloaded WhatsApp, the "last compatible version" trick will not work. whatsapp old version for ios 9.3.6 download
A: Yes, text messages (SMS-style chats), images, and basic voice notes work. Group chats work, but reactions and polls may appear as text. Short answer: Yes, but only if you previously
Do not download "IPA files" from random websites. Those are often malware. Use Apple’s official legacy download system. Short answer: Yes