What year is it? Is TAS considered canon right now or not? It seems to change every year or so.
TAS was canon (I guess) from 1974-1986.
Gene Roddenberry declared it non-canon in 1986, though I don't think anyone knew about that until 1988. (There was a much less developed sense of "canon" back then, with only 79 episodes + movies to deal with). TAS remained indisputably non-canon from 1986 until 2003. This was reinforced by the principal reference work of the time, the
Star Trek Encyclopedia.
In 2003, Memory Alpha launched, and it quickly became the most important
Star Trek canon resource on the planet, to the point where actual production staff regularly used or even contributed to it. It was (and is) famously stringent about its canon policy, which helped keep the resource clean and usable for everyone. Except for one thing: from the very beginning of its existence, Memory Alpha has considered TAS canon. Why?
Because it wants to. That's it. M-A knew TAS was non-canon, but counted it as canon anyway, just 'cause it could.
When you're the most important canon resource on the Internet, that's a big old deal.
From 2003-2006, the canonicity of TAS was increasingly disputed. Public pressure on CBS to canonize TAS mounted... and, with the release of the TAS DVD set, Official Star Trek caved.
Star Trek was at its nadir, with no
Star Trek on the air and the Abrams movie hadn't been announced yet. Canonizing TAS was about the only thing you could do to generate excitement about
Trek in 2006.
There were a couple more years of people questioning whether the StarTrek.com acceptance of it was "reallllly" canonizing it, but that ended by, oh, probably 2008.
TAS has been canon ever since. People have been
surprised to find that TAS is canon, and people have been
annoyed that TAS is canon (it took me a decade to warm up to it), but CBS has been pretty consistent that it is.