A majority of Wikipedias now have access to the Growth features. The Growth team has published an FAQ page about the features. This translatable FAQ covers the description of the features, how to use them, how to change the configuration, and more.
The new version of MediaWiki will be on test wikis and MediaWiki.org from 14 September. It will be on non-Wikipedia wikis and some Wikipedias from 15 September. It will be on all wikis from 16 September (calendar).
Starting this week, Wikipedia in Italian will receive weekly software updates on Wednesdays. It used to receive the updates on Thursdays. Due to this change, bugs will be noticed and fixed sooner. 
You can add language links in the sidebar in the new Vector skin again. You do this by connecting the page to a Wikidata item. The new Vector skin has moved the language links but the new language selector cannot add language links yet. 
The syntax highlight tool marks up code with different colours. It now can highlight 23 new code languages. Additionally, golang can now be used as an alias for the Go programming language, and a special output mode has been added to show a program's output. 
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. 
Changes later this week
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.