The AdvancedSearch project originates from WMDE's Technical Wishes project. More info can be found in German and in English.
It is now in a usable state, with some more functionality to come (e.g. deep category search). For giving feedback on the interaction and testing the already implemented keywords, it should become a beta feature on de-wiki.
Update: The ar-wiki community also requested a beta feature deploy, see T180291: beta test AdvancedSearch in Arabic Wikipedia
It is planned to be on dewiki and ar-wiki on the 29th Nov 2017
It is planned to make AdvancedSearch a beta feature for all wikis at the start of 2018, and to take it out of beta towards August 2018.
The AdvancedSearch project aims to make visible what search help exists already. It adds a form to the Special:Search page, where keywords and other search tools can be used through input fields.