In 1989, due to ST IV and TNG, Gene Roddenberry had some influence on Paramount when the tie-in licenses were being renegotiated. All proposals and manuscripts had to pass by then-Viacom Consumer Products (and now replaced by CBS Consumer Products) and his own Star Trek Office. Richard Arnold, a longtime volunteer tour guide and convention volunteer, began a fulltime position, as Archivist, and was soon vetting tie-ins on GR's behalf, and the Office began distinguishing what was "canon".
At first, it was defined as "live action, as screened", and that which was produced by Desilu/Paramount/Viacom. That cleverly shunted out TAS (which had been produced by Filmation and Norway Corp. for NBC Children's TV, and wasn't live-action), and licensed tie-in books, comics and RPG materials, but the definition also omitted new live action material, even when filmed on the actual sets (i.e. for video board games, computer games and Universal's "A Star Trek Adventure"). By extension, it also covered later products, such as the two live-footage interactive rides at the "Star Trek Experience" in Las Vegas - and perhaps(?) restored/new footage in revised videos of the movies and episodes.
Roddenberry passed away in September 1991, Richard was suddenly no longer Star Trek Archivist, so by default the "keeper of canon" passed to Rick Berman. Paula Block and her team at Viacom, (later CBS) Consumer Products, continued vetting the tie-ins. Essentially, nothing changed, although TAS started being referenced in the tie-ins again, and was retro-added to the official online encyclopedia entries.
After Berman left, nothing ls has really changed. Bad Robot would designate any change to accepted "canon", and Orci and Kurtzman were supervising how IDW would approach any TNG- and TOS-related tie-ins to the 2009 and 2013 movies (eg. "Countdown", "Nero", "Spock Reflections", "Countdown to Darkness", "Star Trek Ongoing", etc).
But not much has changed. "Canon" would still be "as screened", and that which is produced by Desilu/Paramount/Filmation/Viacom/CBS/Bad Robot. It has never been the novels, comics or RPGs. No one ever clarified that changes for DVD releases altered canon. (e.g. Was the Vejur cloud 82 or two AUs in diameter?)
For the last few years of her tenure at "Voyager", Jeri Taylor treated her novel, "Mosaic", as canon, but only because she was showrunner, and was using Janeway background material she had conceived of for the episodes if ever needed. She invited the other writers to use that book, and "Pathways", but they started ignoring the VOY hardcovers as soon as Taylor left the series.
"Canon" really only affects the licencees anyway.