Starting from 4.0 version, a temporary solution has been implemented to resolve the problem described in SERVER-42737, i.e., the catalog refresh on secondaries stuck waiting for a notification which not arrive (see this analysis for further details).

The original idea was to get rid of this solution after implementing the catalog loader writes with transactions (see SERVER-44105, closed as "Won't Do"). Nonetheless, the current plan is to remove the ShardServerCatalogLoader (see PM-2948), which should allow to get rid of the temporary solution mentioned above.