See, for example, http://wiki121.countingopinions.com/ vs http://wiki.countingopinions.com/ -- especially the Template:Announcements page on that wiki. On 1.19 it is rendered as http://wiki.countingopinions.com/index.php/News:Announcements and on the 1.21 installation it is rendered as news:Announcements%7CAnnouncements.
I'm going to posit that most people use "News:" as a prefix more often for their articles (as MW allowed in the past) and are not even aware that it could be used as a schema for a url.
Version: 1.20.x
Severity: minor
See Also:
https://bugzilla.wikimedia.org/show_bug.cgi?id=41960