about a decade later a book by the same team gets published that does something similar, but is not rejected by Paramount, even though a new movie based on a similar idea is about to hit in the same amount of time.
It's all a matter of timing. The deal with "Federation" was that crossovers in the tie-ins were being actively discouraged at the time, but the storyline was such that the two Enterprise crews never met, so it got approved.
DC Fontana's novel, "Vulcan's Glory" was being written about the same time as rumours/factoids of the storyline of ST V, then in pre-production, were being released. Roddenberry and Fontana were being asked to comment publicly on the "Spock has an older brother?" concept and it seemed they were displeased that DC's old TOS memos about the importance of Spock not
suddenly having siblings in TOS episodes carried no weight with Shatner & Loughery. (They'd probably never read them.)
But "Vulcan's Glory" has a line, from Amanda, IIRC, where Spock is called "the only son of Sarek"
. Years later, I asked DC about whether this was a deliberate stab at the then-not-yet-onscreen ST V storyline with Sybok and she smiled cheekily and said, "I'm glad you noticed."
Had the book been commissioned after
ST V was out, that line would have been edited out.
back around 1985/86, before TNG was announced, proposed a story that would've had Kirk and crew travelling a hundred years into the future, but the proposal wasn't approved because of TNG.
But... it was possibly also because David Dvorkin's "Timetrap" was already in train. That one had Kirk apparently timeslipped into a future where the Klingons were the UFP's allies, IIRC, and it came out in June 1988. I recall, when reading it, that I'd hoped Dvorkin had been able to slip in a few TNG-type references, but there were none.