The core server is deprecating the system.js functionality in 8.0, customers should be made aware that the system.js may not work with new features and will be removed in a future version of MongoDB.

Description of Linked Ticket

MongoDB allows customers to store javascript functions on the server in a system collection called system.js.

The functions customer stored within the system.js collection could be used within aggregations and queries such as mapReduce and $where respectively. 

Starting in MongoDB 5.0 mapReduce is deprecated in favor of an aggregation pipeline and for $where alternatives such as $expr are preferred due to better performance.

System.js should be deprecated as the native built-in capabilities of the aggregation framework have increased and $function exists if custom expressions are needed.