There is no separate command to create routing.module file. But, that can be created while on the creation of the module:
ng generate module [module-name] --routing
or the shorthand version of the command:
ng g m [module-name] --routing
… will create the module and add the mappings/metadata linkings.
I was searching about this a bit and found some article which has a very good explanation for different kind of commands.
The Ultimate Angular CLI Reference