This is a frequent use case for us: learners are enrolled after the fact and credited for completion. We find ourselves having to turn messaging off all the time as a result, and then quickly back on again so that learners in future sessions get their messaging. Or, admins go ahead and do it, and learners wonder why they got a confusing message. If we could have an overall setting for the course that if people are added retroactively they don't receive a message, that would help us a lot.