There have been other versions of this idea that have been flagged as "implemented", (Support Related Records in ArcGIS Online Pop-ups and Format Related Records in pop up ) but the ability to show or format related records in the pop-up still does not exist.
It would be great to show a list of the related records in a feature's pop-up. I'd like people to be able to view different contact information when clicking on a building, but the only way for this to work currently is by creating a long one-to-one join and publishing that as a service or by forcing users to open the table. Both have implications with data design and user experience.
Both of the previous ideas where marked as implemented, but the 2016 update to ArcGIS Online only added related table functionality in the webmap's Details Page (which is not a user friendly way to interact with a webmap). The 2016 update also allowed the ability to summarize related data (Configure pop-ups—ArcGIS Online Help | ArcGIS ), so one-to-many relationships with text display a feature count. Again, while an improvement, this isn't terribly helpful for those wanting to view one or two details stored in a related table.
I understand it may be difficult to display potentially complex relationship information in a small pop-up, but I have faith in ESRI finding a way to make this better if they continue to work on it.