https://tools.wmflabs.org/globalsearch exists, which we weren't aware of when we build the new global search tool at https://tools.wmflabs.org/global-search. The former is from 2014 and apparently queries each wiki one by one. The new tool is powered by CloudElastic which is dramatically faster and more accurate. The new tool is also localized.
The old version features a sub-tool that does external link searches, but this can be accomplished in the new tool too (just search for the URL as you would any keyword).
Finally, the old tool features Global Page Text, which shows the (partial?) content of a page with the given title, for each wiki that it exists. I'm not sure how often this is used, but we can keep it as-is if we want. It didn't work but so well for me; for instance I did not get any result for enwiki when I searched for User:MusikAnimal.