1. The client is up-to-date on GitHub, which is a much better situation than with WhatsApp or Telegram. An outdated server code just gives you reason to distrust their metadata handling.
2. Trust is a bit more complicated than that. FB sells your data and Telegram is very shady in many more regards. The situation with Signal is completely different.
3. Even if the server code on GitHub was updated, what exactly would give you reason to believe that they run that version in production? Ultimately, you cannot trust the server. This can only be solved with decentralization.