Go to Settings > General > VPN & Device Management → tap your Apple ID → Trust the developer profile.

Some developers maintain “Decrypted IPAs” collections. Search for decrypted-facebook-ipa on GitHub. Read the README carefully — many are taken down due to DMCA.

Instead of hunting for an old Facebook IPA, consider — the official lightweight version designed for older devices.

While Apple doesn't host a public archive of old IPAs, there are several ways to retrieve them. 1. The Official App Store Method (Safest)

Without a valid signature, the app will crash on launch.