Gene Roddenberry (or, probably, Richard Arnold) made statements about what's canon.
At a time when he had been eased back to a ceremonial position and had no real authority over the series, which is probably why he felt the need to compensate by asserting the right to define canon. The people actually creating canon don't have to define it, because they're too busy making it.
But that was an exception that fandom misconstrued as the rule. Generally the only time anyone in authority has reason to address canonicity is in regard to secondary or peripheral productions whose status is ambiguous, such as tie-ins or works in other media, e.g. animated versions of live-action franchises, TV series set in movie universes (e.g. Netflix Marvel), or in this case, audio dramas. There's no need to define or declare the main canon as canonical, because that's just what it is by its very existence.
I mean, maybe Paula Block or John Van Citters has said something, but they were/are in licencing. They had no control over the TV series or movies.
Their job is only to make sure that tie-ins are consistent with screen canon, not the reverse.
I don't know if anybody specifically made that decision, it's pretty much the standard that tie-ins aren't canon. Typically the only time you have someone making a specific decision is when they decide a tie-in is canon. And even when they do, it doesn't really mean that much because even "canon" tie-ins are contradicted all the time.
Yes. It's more common these days for tie-ins to be nominally canon, but almost inevitably, they end up getting contradicted by new canon, or at most only approximately followed. For instance, the comics based on the Legendary Godzilla/King Kong MonsterVerse are supposedly canonical, and monsters introduced in them have shown up in the
Monarch TV series, but the character of Lee Shaw in
Monarch is only loosely based on a character from the first tie-in comic and the details of his biography contradict the comic.
Just look at Star Wars, after the Disney sale made a big deal out of how everything is canon unless it specifically made to not be, and even they've had quite a few contradictions come in between different tie-ins and between the tie-ins and the Disney+ shows.
It amazes me how quickly people forgot that this was also true of the pre-Disney "Expanded Universe" that began in the 1990s. Lucasfilm Licensing insisted all the books, comics, and games were canonical, but George Lucas himself was on record as disagreeing; new productions often cribbed characters, species, nomenclature, etc. from tie-ins and toys, but just as often contradicted their continuity, which required the supposedly "consistent" EU to disregard the stuff that had been contradicted while still pretending to be a consistent whole, or come up with handwaves to reconcile the contradictions.
And it wasn't the Disney sale per se that led to the replacement of the EU with a new tie-in continuity. It was the making of the sequel trilogy, which covered much of the same time frame and conceptual ground as the EU fiction but in a different way, so that it was no longer feasible to continue the EU storylines and they had to start over with a clean slate consistent with the new canon. Basically the same as how the post-
Star Trek Nemesis novel continuity was able to keep going until
Picard canonically established a contradictory version of post-NEM events.