The feature to contact other users has been requested many times before. There is even a protected question on the issue! Valid reasons against such a feature include not wanting SO to morph into a social media site, complete with abuse and echo chambers. I agree with this!
This request is for the feature under tightly defined conditions, specifically when another user has performed a public action (such as leaving a comment) and time-limited (eg. 7 days since last public action). If abusive or non-constructive, recipient can flag for moderators. If moderators agree, user can be banned from messaging for X days or require additional reputation.
To be clear, automated bump messages would be fine. Something of the form "User XYZ has edited question ABC that you [DELETE-AS-APPRORIATE] [voted to close] [put on hold] [gave an answer to] [commented on]. Ignore / visit question."
A lot of comments assert this feature is somehow intended as a back-door abuse mechanism. Nothing could have further from the truth! This was intended as an entirely positive contribution.
I've since been directed to this related request which more or less is exactly what this feature is about. Only comment is the feature request should also cover "on hold" questions, and other public actions as well. If its automated, maybe even up/down votes as well.
Curious how this has -12 and that one +42 votes (at 2017-06-21T10:33, that is)!