This request was brought up to us by Daniel Javorszky from a customer who would like to know how to add a custom listener to the MessageBus on DXP. The customer found documentation for 6.2 EE, but documentation for DXP does not exist.
Similar blog posts have been made by David Nebinger on How to send out user notifications and how to add scheduled tasks, but no documentation regarding adding a custom listener to the MessageBus could be found.
Based on the two blog posts mentioned above and an example from the portal, Daniel created a proof-of-concept portlet that adds one listener to the "liferay/background_task_status" endpoint and shared that with the customer on the ticket. Unsure whether or not that is the "official" way to go, Daniel brought this up to us and I created this LRDOCS ticket for him.
Let me know if anymore information may be needed. Thank you!