Google has now seemingly stopped selling the Pixel C tablet altogether. The company has yet to make an official announcement regarding this, although the Pixel C has now been removed from the Google Store, including its page. Essentially confirming the discontinuation of the product. In some respects this will not come as too much of a surprise as in spite of this being a 'Pixel' branded product, it did come to market some time before the new wave of Pixel-branded products came through.
As a result of being announced well over two years ago, it could just be the case that the Pixel C has simply and officially ran its course. Of course, there is the added element that this is an Android tablet and generally speaking tablets are not considered to be as big of a seller as they once were. So in addition to the general age of the Pixel C, demand for this particular tablet may have also dried up and further impacted on Google's decision to fully remove the product and its listing. Especially considering that this is by no means an affordable tablet. While there was a slightly more affordable version previously available, Google did stop selling that one back at the start of 2017. Leaving only the more expensive model available since then, which is the one that has also now also been retired from the Google Store.
Another factor that may also be playing into the retirement of the Pixel C is the Google Pixelbook. While the former is a tablet and the latter is a Chromebook, the Pixelbook is the device Google would likely prefer you to buy these days, and in many ways adopts a number of traits familiar to a tablet. Even more so, familiar to the Pixel C, due to the Pixel C's ability to be used along with a keyboard in more of a traditional laptop/Chromebook manner. So it could also be the case that Google now wants consumers to focus more on the Pixelbook which is not only newer but more in-keeping with the current Pixel branding in general. This latter point seems to be further bolstered by the old URL for the Pixel C page listing on the Google Store – which now conveniently points buyers to a landing page for the Google Pixelbook.