It was originally planned for us to merge our indices into cross-project indices in order to add cross-project search functionality (T145918). Over time, that functionality has been analysed, and it appears that merging the indices is no longer a blocker for rolling out cross-project functionality. It's still considered a good idea to merge the indices, but it can happen afterwards.
This task tracks the work to merge the indices together. Most of the tasks in here were removed from the epic for rolling out cross-wiki search functionality, since they no longer block that.