This is the biggest problem. When you open WhatsApp 2.12.222, a pop-up will likely say:
: Older versions lack critical security patches, leaving your personal data vulnerable to malware or hacking. Where to find the APK (For research only) whatsapp for android 2.3 3 apk download old version
and the re-verification of existing accounts on Android 2.3.7 and older. Server Obsolescence This is the biggest problem