Also, calling out @Sybren A. Stüvel (sybren) since he appears to be the resident alembic dev...

This is working as designed; only when the 'Auto Smooth' checkbox is enabled, custom loop normals are written to the Alembic file. However, this wasn't described in the manual at all, so I made an addition to that to clarify this behaviour.

@Sybren A. Stüvel (sybren) Thanks, but can there please be some additional clarification on this?

With fbx export, the process to export something such that the normals come back in properly is:

1. Open a blend file.
2. Export an fbx
3. New scene, Import fbx

Result: Normals match the original blend file

But it doesn't work this way with abc, and auto smooth doesn't seem to really resolve the issue from what I'm seeing so far. More concretely, if you do the following:

1. Go to https://www.blender.org/download/demo-files/
2. Download the Architectural Visualization scene.
3. Open the scene
4. Export scene as abc
5. Create a new scene then reimport the exported abc.

How do these steps need to change to achieve all the same normals that were in the original blend file? I've tried a number of things, among them:
Set auto smooth with angle 0 on all objects -- the shelves in the scene are correct, but anything that had some smooth normals of course come back in faceted. I'd somehow have to "guess" the right auto smooth value for every object in the scene prior to export, or something?

For posterity, the solution to preserve normals within an abc such that they match the blend file is to do the following prior to export:

for obj in bpy.data.objects:
    if obj.type == 'MESH':
        obj.data.use_auto_smooth = True
        if not obj.data.has_custom_normals:
            bpy.context.view_layer.objects.active = obj
            bpy.ops.mesh.customdata_custom_splitnormals_add()

It kind of feels to me like this should be a part of the exporter, at least behind a checkbox or something? The different exporters should have a consistent behavior with respect to "normals idempotence"... fbx seems to have this property whereas abc does not, unless you massage the scene yourself first by adding custom splitnormals layers on everything and turning on autosmooth. I thought that's what the "Normals" checkbox in the abc export option actually does...

@Sybren A. Stüvel (sybren) Thanks! Makes sense, I think that's a valid approach. Though one downside of saving face varying for every object in the scene -- even for objects that have smooth normals -- is that it could significantly increase the size of the output abc file. Another potentially bigger problem, is that upon re-import, if the tool ingesting that abc is not smart enough to merge identical face-varying normals for a vertex into a vertex-varying normal, that "normals bloat" could make its way to the GPU and really hinder render performance. Imagine a scene filled with thousands of smooth-normaled spheres. Exporting that and reimporting that -- even with normals -- should ideally not result in face-varying normals -- which consume many times the space of vertex-varying ones -- making their way to the GPU memory (or cpu memory if dealing with a cpu renderer).

Some other dcc exporters make one slight tweak to your approach: They examine the normals of each object, and if an object has 100% smooth normals (i.e. they can be trivially recaulculated by an ingest tool via averaging the normals of each vertexs' connected faces), normals are not saved for those objects at all. For those objects that have normals that can only be represented via face-varying, those normals are saved to the abc as face varying. The rationale for this approach is that it keeps the file size as small as possible -- The downside is that the tool ingesting the abc needs to be able to generate smooth normals if necessary (using the connected face averaging approach mentioned above). I think this is actually fine tradeoff. Something to consider.

Useful References on this topic:
https://groups.google.com/d/msg/alembic-discussion/xmP2QMlIWKM/-chYFSPWU_UJ
https://github.com/alembic/alembic/blob/master/maya/AbcExport/MayaMeshWriter.cpp#L659

For such an approach to work and allow blender to have "normals idempotence" with abc files that it outputs and re-imports, blender would need to be (optionally?) capable of generating smooth normals on the fly for objects that don't have normals specified, whenever an abc is imported. Maybe it already has that, but I'm not sure offhand...

@Andrew (ajohnson223) please check T71246 and see if you're happy with my proposal ;-)

Looks great! :) Thanks.