From what I can tell the sites are not set to Maintenance Mode when updating plugins as of today.
This is risky as the updater moves the old plugin version to the temp folder (or in other ways make it unavailable/break), and thus if the site is browsed or running any other actions at that time who calls the plugin being updated, it will fail and potentially create issues. On busy sites you'll often find fatal errors logs for the update times.
There's a reason why WP have the auto activation of Maintenance Mode when updating. ;)