Thanks Takayuki for replying.
In such case, it should be documented in some other place. I only stumbled upon it by doing the global search in the plugin package, and somehow discovered it in the release notes.
Other than that, when you check the release notes here, it says that wpcf7beforesubmit is a Custom DOM Event, with a link to the page listing all Custom DOM Events. You cannot find wpcf7beforesubmit on the list and so it’s somewhat inconsistent.
