Aggregation commands sent from mongos to shards have let and collation parameters added to them. This causes those queries to fall back to classic, arguably unexpectedly when the original command does not have those fields set. Interestingly, the equivalent find commands do not have those parameters.

We should update the fallback mechanism to allow queries with simple collations and unused let variables to go through CQF. This will allow sharded aggregations to use CQF.