I think it was only ever common for this assembly kind of work, maybe other businesses shut down as well when suddenly, 90% of the customers wouldnt show up, but otherwise it wouldnt really make sense.Up until the early nineties I believe it was common for certain types of workers (the assembly line kind, I'd say) to have the month of August off because most major companies used to shut down production completely in that period.
It's probably a rumour from back when german car factories employed tens of thousands of people, not only a few thousand like nowadays. After all, several thousand italians came to Wolfsburg alone in the 60s only to work in the Volkswagen factories. Those times are over.. not sure anyone still has company holidays