The new $system.external interface (aka "InterSystems External Servers") is used to call/integrate with external language code/libraries, it replace the old gateways (for example Java gateway) that required the import of proxy objects.
Note that the documentation for the old gateways has been removed from IRIS, so only the new $system.external interface is somewhat (i.e. insufficiently) documented.
Regarding the migration from old gateway to new $system.external interface the documentation says:
Happy new year! I’m Raj Singh, InterSystems’ product manager for Developer Experience and I’d love your feedback on how you use IDEs today and your thoughts for the future.
By update