Thanks for raising this. I've also noticed the same thing, but don't find it to be a concern.
You've asked a few questions, so I'll provide my brief views below:
Should other users/moderatos proactively edit the post to hide the
Should the original author be notified about the visible serial number
and be requested to take corrective measure(s)? (Deleting and
re-posting for e.g.)
No. I don't think we should be telling them to take corrective action for something that in reality poses very little (practically no) risk. I guess I could see an argument (maybe/maybe not) for letting the OP know their serial number is visible in case they weren't aware, and that should they have a particular concern with the idea of their serial number being visible, explain that they can choose to edit their post.
Should the moderators be flagged to remove the serial number from the
By users generally, no. By the original author? Yes, but only if they're concerned about it, no need otherwise.
Should serial numbers be redacted?
No, unless the original author has requested a redaction.
What harm does it cause to leave serial number in visible text?
I think this should be dealt with as a separate question as it's potentially opinion-based and could invite lots of views/answers. I also don't think it should be worded as "What harm does it cause", but rather "What harm could it cause". [I think bmike is going to post a separate question on this].
Without going into details here, my answer would be to say that there's essentially no harm, but in an absolute worst case scenario there are a couple of possible ways someone could try to use a serial number (but not in a way that causes harm to the original author). In fact, the only circumstances I can envisage a serial number being manipulated in a way that could cause harm to the original author involves someone who is both familiar with some of Apple's internal processes and also has access to personal details about the original author. An extremely low risk in the context of our site.