$wgEchoCrossWikiNotifications does not exist in 1.27, only in master. I'll see if I can backport it.

Thank you for your comment. That explains it. Backporting this will be marvelous and cheerfully appreciated. :) I makes sense to me doing it since MW 1.27 is a LTS release.

Backport: https://gerrit.wikimedia.org/r/295113

There's a lot more work on cross-wiki notifications that was done after the 1.27 cut, but I decided not to backport an entire month's worth of commits.

In T138132#2392005, @Catrope wrote:

There's a lot more work on cross-wiki notifications that was done after the 1.27 cut, but I decided not to backport an entire month's worth of commits.

This is utterly acceptable. Avoiding the option on "Special:Preferences" is however a nice thing to do. For people who would like to use cross wiki notifications it will be feasible to move on to 1.28+ etc anyways.

Thank you very much for your backport!!!