I wonder how that got past 'oversight' ?
Rather, they predated such oversight. It wasn't until around 1989-90 that Roddenberry and his people (meaning Richard Arnold, mostly) began restricting what the tie-ins could do. Yesterday's Son
was published in 1983, Time for Yesterday
Besides, giving Spock a son didn't really alter the series status quo that much, because Zar lived in the distant past.
And it was a big success at the time. Not counting movie novelizations, I believe Yesterday's Son
was the first Trek novel to hit the New York Times Bestseller List.