The Shadow Work spread is the most recent kind of layout in this atlas, and its vocabulary comes from outside tarot entirely.
Where the word comes from
The shadow, as a term for the disowned parts of a personality, is Carl Jung's, and belongs to analytical psychology. It entered tarot writing through the wider twentieth-century movement that reread the cards psychologically rather than predictively, and spreads built explicitly around the term are more recent still. No first author can be identified for this layout, and the atlas records the authorship as unknown.
A borrowed word carries assumptions
Using the term commits the reading to a particular model of the person: that there is material which is genuinely hidden from the querent, that it can be surfaced, and that surfacing it is beneficial. Those are claims from a school of psychology, not from the cards, and they are contested within psychology itself.
The distinction that matters
Spreads of this kind sit close to territory that belongs to trained clinicians, and working readers are generally explicit that a card reading is not therapy and that a reader is not a therapist. That boundary is part of how the practice describes itself.
This atlas describes the practice as its practitioners describe it. It does not instruct anyone in it, does not present it as treatment, and records the positions of the layout separately from this history.