Opting not to update means you cannot add speakers, services or voice assistants to your Sonos system and that you will not receive new features or security patches. If you want any other information for sonos setup you can go to official support site.
If any smart devices in your home have the Sonos app installed on them, you will need to disable auto-updates on them to ensure they do not update to the latest version. If the Sonos app is accidentally updated, then your Sonos system will also need to be updated in order to work with that device again. It is not possible to remove an update or downgrade your Sonos system.