The section titled "Pipeline Operators and Indexes" from https://docs.mongodb.com/manual/core/aggregation-pipeline/#pipeline-operators-and-indexes should be updated due to this change. It currently lists $match, $sort, and $geoNear as eligible to result in index use. However, this list is not exhaustive. Due to this change, a pipeline with a $group at the beginning can use an index, even if there is no $sort stage.

In addition to documenting this new behavior, I suggest that we change the language of this page so that it does not claim to be exhaustive. That is, it should not say "Stages X, Y, and Z can result in index use." Instead, it should say something like "MongoDB's query planner analyzes an aggregation pipeline in order to determine whether indexes can be used to accelerate the operation. For example, an index can be used for filtering if a $match is at the beginning of the pipeline, or can be moved to the beginning of the pipeline by the optimizer. Similarly, a $sort at the beginning of the pipeline can be computed by scanning an index in order. As a final example, $group stages which obtain the distinct values of a field can use an index for the distinct operation if they occur at the beginning of the pipeline."