Yes, Map Viewer Classic in ArcGIS Online supports join and relates. ArcGIS Online has the Join Features function as an analysis tool since the December 2016 release of ArcGIS Online. Refer to ArcGIS Online: Join Features (Map Viewer Classic) for more details.
Meanwhile, ArcGIS Online can view or edit layers with related records.
To access the Join Features tool:
If a layer contains related records, the attributes can be edited with the correct privileges, as explained in ArcGIS Online: Edit attributes and related records. These related records are represented as columns with italicized headers on the right side of the attribute table as shown in the image below.