I made a request back in 2017 as part of 2017 wishlist, but it didn't garner enough votes to be in the top 10. Then I realized that I don't have to depend solely on wishlists, do I? Rather than having only centralized settings of crosswiki search results, I think individual users should decide themselves which results to display and which results from any one of sister projects to display in various local projects, like Wikipedia. Furthermore, individual users can prioritize which results they want to first see in mind. I think adding the crosswiki search result settings in the "Search" tab would be best suitable. If not, then where else within the user preferences?
One thing for sure: we must set default settings for various projects in order to reflect and respect longstanding consensus from local communities, like English Wikipedia, who decided to have results from a couple or a few sister projects disabled (e.g. T163463, T171803, T185250)