I have just completed upgrading ArcGIS Enterprise to version 10.6.1 (Server, Portal and ArcGIS Data Store). I tested the CSV drag & drop using a web map in Portal for ArcGIS, and it works perfectly. However, our staff only use Web Applications (i.e. not Web Maps) as these are much cleaner and look better, especially for staff who rarely see spatial data. But when I try to drag & drop a CSV file onto a Web App...nothing happens.
Is there something else I need to configure to get this to work? Everything else seems to work, except this.
Judging from this idea, I'd say that's not implemented.
Another solution is to set up a GP service within a GP widget. The GP service could take a file as input and use the Make XY Event Layer tool to create XY points from the data. Once that data is returned by the GP service, it'll display in the web app.