How would we add Drupal Core modules as dependencies on custom modules post 8.6.2?
If you're referencing a recent change in a core module, it was a style/formatting change. 

Adding namespaces for dependencies has been in core for a while. The {project}:{module} format is a better practice for removing ambiguity in the event of module/project machine name overlap, but both will work. 

For documentation reference, see Let Drupal 8 know about your module with an .info.yml file under the example section for dependencies:


  
    dependencies - A list of other modules your module depends on. Dependencies should be namespaced in the format {project}:{module}, where {project} is the project name as it appears in the Drupal.org URL (e.g. drupal.org/project/views) and {module} is the module's machine name. Dependencies can also include version restrictions, for example webform:webform (&gt;=8.x-5.x). Note that if your module has dependencies on other contributed modules or libraries, these should be declared in the module's composer.json file.