Will it be nice to have the above function in the Webmap in ArcGIS Online?
I searched around and found this article in ESRI Knowledge Base:
but, contrary to what stated in the article, I found join via relationship class does not work in ArcGIS Online, it only works as a relate.
A unique feature of join is the appended fields which can be used to symbolise the features and label the features. there are no appended fields in the service in ArcGIS Online, so I don't think the genuine join is working.
There are some ideas in ArcGIS Ideas, but I don't know if ESRI is considering the fuction. let's promote them anyway if you think it's a useful function.