This is always the safest, most up-to-date, and feature-complete option.

Furthermore, downloading a or modded Messenger IPA is explicitly illegal in most jurisdictions because it circumvents the app’s security measures.

Warning: This method often relies on enterprise certificates that Apple may revoke frequently. ⚠️ Critical Risks and Warnings

Once AltStore is on your phone, you can "sideload" the IPA file directly from the app.

This is the safest method because you are using the official app from Apple’s servers. You’ll need a Mac with Apple Configurator 2.