Actually, it was around the time of TNG that GR and/or Richard Arnold requested TAS not be considered canon, meaning its events need not be taken into account when writing new scripts, etc. That's no great shock as GR was tossing aside chunks of ST at the same time and tightening down across the board on what Trek was in order to shape TNG as he saw best. At the time of TMP, GR was still speaking favorably about TAS as evidenced by his comments in The Making of Star Trek - The Motion Picture. The series wasn't every thing he hoped it might be but he still felt it was a worthy effort. Why else would he have suggested, as has been mentioned on this board, Majel make a cameo as M'ress in TMP (she balked at the make-up)? In 1979 and until 1987,TAS was a much Trek as anything else.Inclusion in the official timeline at Roddenberry's insistence, the various TAS references in DS9 and ENT, as well as the DVD release, say otherwise.
Since, as I understand it, it was GENE who, around the time of the first movie, declared TAS no longer a part of the timeline, what "official timeline" do you mean, and where did you hear he "insisted" on it being included?
TAS may not be canon, as Paramount defines it ("keep this in mind while you're writing"), but neither is it officially excluded from the continuity, as evidenced by the numerous mentions in subsequent series. TAS can easily fit into the overall Trek continuity since that covers "Stuff that happens in the episodes and, by and large, is never spoken of again" as there are only a few points of conflict and only one truly major one (TFF versus "The Magicks of Megas-Tu").By the way, I agree with the others. Certain "historical" incidents being mentioned onscreen that happen to be in step with TAS don't make ALL of TAS canon. 'Sorry. It doesn't work that way.'
The two sides of this debate are need to learn to live and let live. Those with a less than favorable view of TAS have to acknowledge it aired and has had a lasting influence, to a greater or lesser degree, on Trek overall. The stories are no sillier than some shown in live-action episodes ("Spock's Brain", "One Little Ship", "Innocence") and several are very strong. Being a TV series produced by D.C. Fontana, supervised by GR, featuring the original cast, and with episodes written by many ST scriptwriters, it deserves not a little respect. The intent was definitely that the show was Star Trek.
Likewise, the TAS advocates must accept the series has a rushed and rough quality that impacts the stories and more significantly, the animation. That presentation is off-putting to many who simply can't see past it. Its official status, alternately rejected and tolerated by the production staffs and CBSmount PTB, is always going to depend on who's in charge and it will never have the full credibility of the live shows with a significant portion of the fandom. Unless and until a production team comes along that is willing to embrace and explicitly include it as part of the greater whole, the chances of which are increasingly remote, TAS will remain the bastard red-headed stepchild of Trek productions.