Section "Format" talks about "fields that are used as keys in a map":
All field names in the specification are case sensitive.
This includes all fields that are used as keys in a map, except where explicitly noted that keys are case insensitive.
The Overlay specification so far doesn't have any of these maps, the sentence probably was copied from the OpenAPI specification, which defines several maps with "free-style" keys.
Should we keep this currently effectless statement in case we add such maps to the Overlay specification?