I think this is good idea.

In a vanilla Multisite install, would site admins or the primary admin email address be emailed about this? I wonder if you can tap into that email and send it to support.

How strict are our policies on adding plugins to the network? There’s a plugin called pending-submission-notifications , which does just that but hasn’t been updated in five years. Reviewed the code. It’s simple and looks good to do the job.

I wouldn’t want to code this feature from scratch since there’s one already made.

@pkevan suggested that Edit Flow, an already installed plugin, might do the job. If I'm not mistaken, with Edit Flow, users would need to actively follow certain posts, after which all status changes are posted. If that's the case, I'm afraid it won't solve this.

If Edit flow doesn't solve this (require investigation), then a simple code addition on transition_post_status would probably suffice and would be preferable to another plugin.