“Supposal” is a play on the words supposal and proposal. Writers often take this liberty, assuming the reader will forgive the obvious error but appreciate the descriptive nature of the word.
For example, if I were trying to describe the multiple attacks on my conditions of service, a made up word like “supposal” just might neatly illustrate the employer’s perception of the iron clad nature of my conditions of service....given the inadequate labour protections in place.