In my organisation, I found that messages posted in the News Forum DID go out to students, even when the course was Unavailable.
- Students were already enrolled.
- Students messaging preference for forums was "Digest".
- The Forum was visible.
- Forum subscription was "Forced"
- Course availability was "Hide"
The normal cron processes on Moodle only send out messages created today to students who are currently enrolled. Later enrolments don't get a flood of email: they have to read the Forum, to catch up on what has already been said.