Facebook abandoned support for iOS 9 years ago. If you try to download Facebook from the App Store on an iOS 9.3.5 device today, you will likely receive an error message stating that the app requires iOS 13 or higher.
For many users on iOS 9.3.5, this is actually the superior long-term solution because it will never break. The IPA method is for those who want speed, notifications, and the classic app UI. facebook ipa for ios 9.3.5