I was digging around trying to find if the Trace Tool exists in AGOL or not? I was happy to see there is a snapping functionality but tracing would be much better for polygons with lots of vertices.
I think this functionality is not yet available for editing in ArcGIS Online. You might like to post this idea at ArcGIS Ideas.
Linking your Idea page ArcGIS Online Trace Tool
Even you should vote up.
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