Jailbreak your iOS 9.3.5 device using Phoenix, install AppSync Unified, and use a v189.0 IPA. And keep a Safari bookmark to mbasic.facebook.com as your backup. The legacy iOS community is strong, but even we must admit: 2018 was a very long time ago.
Third-party Facebook wrappers like "Friendly" still have last-compatible versions (v5.0.6) for iOS 9.3.5. These are often better maintained than Facebook’s own IPA. Search for "Friendly IPA iOS 9.3.5". facebook ipa for ios 9.3.5
Look for . Version 184.0 (late 2018) is widely reported as the final stable build for iOS 9.3.5. Jailbreak your iOS 9