Turn off automatic updates in Settings > iTunes & App Store. Also, do not delete your WhatsApp data. If you lose the installed old version, you may never get it back.
Modern versions of iTunes do not have an App Store. However, Apple released a specific version (iTunes 12.6.5) for businesses to manage apps. You can use this to "purchase" the app on your account so it shows up in your device’s history. Download and install iTunes 12.6.5 on your computer. Sign in with your Apple ID. whatsapp old version for ios 9.3.6 download
No. iCloud backups from WhatsApp iOS 12+ are not backward compatible with iOS 9. Turn off automatic updates in Settings > iTunes
WhatsApp often refuses to send SMS verification codes to versions that are "End of Life." Server Connection: iTunes & App Store. Also