MediaWiki had a feature that would highlight local links to short articles in a different style. Each user could pick the size at which "stubs" would be highlighted. This feature was very bad for performance, and following a consultation, has been removed.
A technical change was made to the MonoBook skin to allow for easier maintenance and upkeep. This has resulted in some minor changes to HTML that make MonoBook's HTML consistent with other skins. Efforts have been made to minimize the impact on editors, but please ping Jon (WMF) on wiki or in phabricator if any problems are reported.
There was a problem with search last week. Many search requests did not work for 2 hours because of an accidental restart of the search servers.
The new version of MediaWiki will be on test wikis and MediaWiki.org from 21 September. It will be on non-Wikipedia wikis and some Wikipedias from 22 September. It will be on all wikis from 23 September (calendar).
The meta=proofreadpage API has changed. The piprop parameter has been renamed to prpiprop. API users should update their code to avoid unrecognized parameter warnings. Pywikibot users should upgrade to 6.6.0.
The previously announced change to how you obtain tokens from the API has been delayed to September 21 because of an incompatibility with Pywikibot. Bot operators using Pywikibot can follow T291202 for progress on a fix, and should plan to upgrade to 6.6.1 when it is released.