This is a common use case - we want to Identify on a DEM raster (imagery) in a webmap and get a popup showing the elevation (the pixel value). I figured out how to do this as in the first link above. But, we need it in the webmap. We like the ArcGIS Online because its popups "just work". Like this... Edit fiddle - JSFiddle
I had already coded an identify to show only visible layers in the popup in a custom API 3.7 viewer, based on threads with Ken Buja et al back in the day, but we are now solidly onboard the Web App Builder train. The workflow we have created now puts all layers in to one service and the DEM imagery is one of the sublayers.
The way we use the webmap popup, is I turn it on for every single layer. That way, if a user clicks on the map, info comes up, for any visible layer. No 'Identify' concept etc. I have found users absolutely love this. It's discoverable and immediately intuitive. I wish the back/forward < > buttons were a little more prominently displayed perhaps, but, I have found after training a few hundred staff that I explain it in a few seconds showing paging between a few different layers that I identify on, and they get it immediately and like it. So, Identify on a raster would be very useful. For elevation, and perhaps categorical rasters like land cover.