He wasn't looking for just any old version. He remembered reading on a forum that was the "sweet spot" for older devices. It was recent enough to support the current end-to-end encryption protocols but old enough that it didn't require the heavy system resources of the newest updates.
: Before uninstalling your current app to install an older version, ensure you back up your chats via Settings > Chats > Chat Backup to avoid data loss. Compatibility He wasn't looking for just any old version