In case the Echo (rECHO) extension is installed on a stand alone wiki or on wikis with no awareness of farmed wikis, i.e. cross wiki notifications are not configured, the user preference option for cross wiki notifications should not show by default. The related system messages are "prefs-echocrosswiki" and "echo-pref-cross-wiki-notifications". In short, even with $wgEchoCrossWikiNotifications = false; which is the default I see these two shown on "Special:Preferences".
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!!!