It is important to note that Facebook Messenger and the main Facebook app are separate.
Installing an APK on older versions of Android requires a few manual steps: Facebook (Android 4.1+) APKs - APKMirror
Search for: "Facebook Lite 78.0.0.19.119 APK" – Verify the SHA-1 signature if possible.
Use this guide to buy yourself 6–12 more months. But honestly, a used Android 10 phone costs less than a dinner out. It’s time to upgrade.