This thread started in Feb, 2022, so almost 2 years ago! I know this is the AGOL forum, but this issue spans AGOL and Enterprise. Last I heard, there was a way to "View" (only) related records in a map pop-up in AGOL Hosted Features. Nothing about editing, adding, deleting, attach child to another parent, M:M and Feature to Feature relationships, etc. in Enterprise still. We are stuck in 10.9.1 and Viewer Classic in part because of this issue. What happened to using relationship classes in an Enterprise RDBMS? geodatabase in web maps/apps?