Destructoid has provided a follow-up stating that the year-long contract for Yaris has expired, which explains the absence. This isn't Microsoft making good on its past plan to start delisting XBLA games or anything. Yaris' time has simply come. Actually, Yaris' time should never have come in the first place, but that's neither here nor there.
Though Destructoid says Yaris has been "completely eradicated from the Arcade consciousness," we verified that the game isn't completely gone yet. Ryan was able to pull up his download history (in the NXE dashboard, no less), find Yaris on the list, and download it again. That will only work if you downloaded the game in the past, though; otherwise you're probably out of luck. But then, other than 200 achievement points, you aren't missing anything. At all.
I truly can't believe I just wrote a story about Yaris. I mean, really?