I was wondering the impact of readConcern "after/atClusterTimestamp" for user collections in "local" database. Then, realized that
user writes to (unreplicated) collections in "local" database (except oplog ) are considered as ordered commits and those writes aren’t timestamped. So, reading those collection data with readConcern “after/atClusterTimestamp” doesn’t have any impact. To my surprise, we don't throw any error when a user provides writeConcern (like majority) or readConcern to "local" database collection instead we silently ignore those read-write concern values and execute the operation.