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The feature document is meaningless and merely an obsolete record. SERVER-57125 did changes to stop creating new feature documents in 5.1. But, we didn't do any work to delete the existing obsolete feature document in future versions, which were generated in older databases. Because of this, unnecessary feature document checks are present in all methods that iterates the MongoDB metadata table (some examples: this, this and this ) . Failure to include this feature document check in those call sites caused serious issues, like, SERVER-85836.
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