Motivation:
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
Time plan:
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.
Background:
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.