If this is the case, then writing 3D coordinates would be very interesting in future releases given the 3D developments in the ESRI JS API 4.x.
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If we look at the format quick facts, it is specified that z-values aren't supported (at the bottom of the page):
Consider creating a new idea for 3D support in the ESRIJSON format and let us know the link so we can vote for it.
Idea posted...