In 8.3, we planned to add support for cross-db $unionWith/$lookup/$graphLookup on unsharded collections. Ahead of the 8.3 release, we discovered a server crash interaction with $queryStats due to previously insufficient testing (SERVER-122323). Since the functionality was not a requirement for 8.3, we decided to remove it from that version but the code still exists in master and is planned to be released in 9.0.

Before 9.0, we should ensure we have sufficient testing (and check for other bugs) so we are confident in releasing this syntax in 9.0