The db/query/stats directory contains code related to maintaining and using statistics for query optimization (originally written for the Bonsai optimizer, but being reused for the cost-based plan ranking intiative). This could be easy to confuse with db/query/query_stats, which contains code related to the $queryStats telemetry feature.

In SERVER-92832 we intend to introduce a new db/query/planner directory. After we do so, it would make sense to move the stats directory to db/query/planner/stats.