Good question. I’ve searched the freelist TCUG archive and did not find it mentioned since the records began in 2001.
The earliest TAL on the DfT site is from 1993 regarding the original Toucan trials. It shows a blank display in photos and diagrams but no explanation why. http://www.dft.gov.uk/adobepdf/165240/244921/244924/TAL_10-93
I assume it is blank as it was easier then deciding on what message and diagram to show before the trials established if a combined crossing would work.