MediaWiki 1.26 is no longer supported as of MediaWiki 1.28's release, November 8, 2016.
This isn't a small change. There are lots of things that need to be prepared to upgrade 7600 wikis to a new major version.
So we're looking at a 1.26 freeze sometime tomorrow evening (25/07/16) and then we'll begin working towards a 1.27 release with an internally agreed, rather hopeful timeline.
We've been running MW 1.28 since 16 December 2016, so this can be safely closed.