Here's a UV sphere; a panel has been cut into it.
These are its vertex normals:
These are its face corner normals:
These are its face corner normals under a Data Transfer modifier from an intact version of the sphere, with no panel cut out.
But an Extrude AltS or Solidify Modifier still results in this:
Is there any way of persuading these operations to use the custom normals, so the panel faces are still contiguous with the rest after extrusion, with no gap? Or some other way of achieving that result?