It’s called from erpnext/public/js/communication.js, which overrides/extends the default frappe communication.js
You wouldn’t need to call the database directly, just add an arg to the js call and then use that argument in the frappe.get_doc call. Honestly, I’m not sure why code related to the issue doctype is being included in the frappe codebase in the first place, but I’m still trying to wrap my head around the Inbox design.