Notes on the approach we're taking on the project page:
We need to fill out the New Notification Data Form for the Collaboration team, as seen on T174388.
For the investigation: What's the basic technical approach? Any hidden dangers?
This is very easy. Let's just do it. Add summary parsing to EchoDiscussionParser:: generateEventsForRevision(). The code parsing edit summaries into HTML lives in Linker::formatComment() and can be referenced for implementation details.
Some discussions may have user page link in section autosummary (e.g. /* [[User:Foo]] is eating my babies! */). While MediaWiki tries to filter such links out of autosummaries, it can still be tricked. We should avoid pinging the users in question every time such sections are edited, so all links between /* and */ should be ignored.
New possibility of harrassment
Because all such pings will be public, the abusers will not have any plausible deniability. Users will have all the usual options of disabling the particular type of notifications or ignoring specific senders.
Should this create a new class of notifications in user preferences, or a generic "Mention" should be enough?