Using ArcGIS Server version 10.7.1 with the old (classic) Map Viewer, relationship classes are functional.
Switching to the new Map Viewer, relationship classes no longer work.
This post says ArcGIS Server 11.2 supports relationships.
So support for relationships was dropped from version 10.7 to 11.1 if the new Map Viewer is used? Maybe they could be made to work with the Map Viewer Classic, but aren't we being encouraged to switch to the new Map Viewer?