[4] was made to figure out common use cases and possibilities to enable them again.
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[4] https://phabricator.wikimedia.org/T215858

I just want to highlight this ^ thing Joaquin said and mention that our team (Data Engineering) is also participating in brainstorming ways to bring back not just cross-wiki joins but better datasets to run these queries.  We have some good ideas, so please do participate in the task and give us more input so we can pick the best solution quickly.