WordNet records no synonym and no antonym for walkout. It is the only member of each of its synsets, which is common for technical, taxonomic and single-sense vocabulary — there is no other English word that means the same thing. The broader and narrower terms below are the useful relations in that case.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
a strike in which the workers walk out
the act of walking out (of a meeting or organization) as a sign of protest
“there was a walkout by the Black members as the chairman rose to speak”
Synonyms, antonyms and the broader and narrower relations for walkout come from Princeton University’s WordNet. Full sources and licences.