I am not writing this in a challenging tone, but a curious one.
You could follow the links in my profile. My name is Ian McLean, I was Vice President, then President of the oldest and largest Star Trek Club in the southern hemisphere from 1983-1992 and our 1000-strong Sydney-based club had frequent correspondence with the Roddenberrys (Gene, Majel and Rod), Bjo Trimble, David Gerrold, Susan Sackett and Richard Arnold. We had everyone except Gene and Rod in Australia for conventions, and Richard has been an
annual guest at a Brisbane convention since 1988. I first met him in 1983. We've also had John M Ford and DC Fontana at conventions Down Under. So we heard lots of "dirt" on the politics of ST tie-ins, from both sides of the barricades.
When GEnie and Usenet started up (pre Internet), we had a member who used to download boxloads of printouts of posts by the ST authors of the day, especially the tense 1989-1991 period, where a Star Trek Office memo was sent to the licensees after all tie-in contracts had been declared null and void (during the hiatus between Seasons One and Two of TNG) and had to be renegotiated.
So many of my novel anecdotes of those days are based on personal memories of those old BBS posts (a few extracts about Andorians quoted in my "Andor Files" blog, old ST newsletters, Roddenberry press releases (Gene used to send letters out to all the clubs who had him as an honorary member), and question-and-answer sessions at conventions.
The Peter David/Richard Arnold anecdotes can be found peppered through the omnibus volume of "But I Digress..." articles by Peter David. Hilarious!
As for TAS references, they didn't start cropping up until the business with Filmation had been resolved to everyone's satisfaction, which unfortunately wasn't until GR had passed (which is why the first ones don't show up until DS9)
The first reappearance of TAS
after the 1989 memo would be Jeri Taylor mentioning the Phylosians in her "Unification" novelization, which ironically, was the TNG episode dedicated to Gene Roddenberry's passing.
But "that memo" was aimed at the licensed tie-ins. The writers of each ST TV series and movie were fully entitled to reference TAS if they so chose.
Effectively, it was never excluded from canon. Canon is material that the current producers choose to draw on as part of the series' backstory.
Yep. It was put "off limits" only to the licensed ST tie-in proposals and final manuscripts between 1989-1991, in a bid to ensure that the tie-ins drew from the live action, as aired, source material.