This started off as a joke commit https://gerrit.wikimedia.org/r/#/c/104789/ but the ability to message everyone on a wiki is a semi-common question that comes up, So it could be a decent feature to consider including.
Version: unspecified
Severity: enhancement
Yeah, and then nobody can come up after a deployment and say "I WAS NOT INFORMED!" :P
Setting this to unconfirmed for now as I'm not sure this is a valid bug.
For small wikis, you can just generate a list of usernames and make an input list using the {{#target:}} parser function. For large wikis, sending a message to every user is probably a very bad idea.
We already have a variety of notices built in to MediaWiki (Anonnotice, Sitenotice, watchlist notices [kind of]) and its extensions (CentralNotice) for the purpose of distributing important information to a wiki community.
Copying Guillaume and Tilman.
I think there are *very few* cases where we actually want to directly contact every possible user on a wiki; the Wikimedia-wide license change a few years ago comes to mind, and I seem to recall that, in that particular case, we sent e-mails to all users using a custom script.
In any case, I don't think we'll ever want to "message" (as in: leave a message on their talk page) every user. What we might want some day is the ability to *notify* every user using the Notifications tool.
It would be less disruptive and invasive than CentralNotice banners, and probably just as effective (and the guidelines for use should be the same as for CentralNotice).
My €0.02.
I'm declining this for these reason: