MediaWiki provides certain user feedback as you reach milestones, or
certain events occur:
"You've created your first edit"
"You were mentioned in ..."
"Congratulations on one year" (I could be mistaken about this one... I know
Discourse does it)
I believe the 'notifications' system  is primarily responsible for this;
while the 'echo' and 'thanks' extensions play major roles. In fact,
read that one or both may be going into core.
Is there a current list to know what messages a User sees? Although the
page at  lists 'edit-thank', is there a built-in system setting for 1st
edit, 10th edit, 100th edit like in the translate extension? Can an
Administrator define their own events (Happy Birthday, Happy Anniversary)?
I'm looking for something more detailed that would list the specific
notifications that users get. I can see the open issues  and project
board , but is there a way for me to tell what new notifications are
just around the bend? It looks like there's active work on "Notification:
Your file was used"  but then again it's been active for 4 years.