Unfortunately, once someone uses “Remove for Everyone” in Messenger (unsend), there’s no built-in way to get those messages back on your end. They’re wiped from Facebook’s servers and can’t be restored—unless you’d already archived them or captured them elsewhere beforehand.
On an iPhone, your best bet is to:
• Restore from an iCloud or iTunes backup made before the unsend happened (this rolls back everything on your device to that point).
• For future chats, turn on automatic iCloud backups and enable Facebook’s “Download Your Information” tool to periodically archive your messages.
Android often touts file-recovery apps, but they’re inconsistent and can risk your privacy. iOS backups are rock-solid and keep your data safe and reliable.