Facebook's authentication servers frequently update. An old IPA might let you type your password, then show an error: "You're using an outdated version. Please update to continue." When this happens, you can no longer log in at all.
If you previously downloaded Messenger years ago: messenger ipa old version new
If you have an old iPhone or iPad that still has the old Messenger installed: Facebook's authentication servers frequently update
If you manage to sideload an old Messenger IPA (e.g., using tools like AltStore or Cydia Impactor), here is what you will experience today: Login – May need to allow “less secure
The Pros:
The Cons (The Dealbreakers):
Verdict: Nostalgic and fast, but practically unusable. Facebook’s server-side checks usually block older app versions from logging in entirely.