In all versions of MongoDB prior to 4.2, dropping an index on a particular collection would cause all active queries against that collection to be killed. See
SERVER-3964. Work done by the query team's "all cursors globally managed" project has improved this behavior such that dropping an index only kills concurrent queries whose plan actually relies on that index. We may wish to document this improvement (possibly as part of the 4.2 release notes?). I've left a few additional details on SERVER-3964, but feel free to reach out directly to me if you have any questions or require additional information.
- Update dropIndexes and its shell helper with the new behavior
- Update the release note with the new behavior