The device was discontinued in December 2017, and the 1.5-year period ended in June 2019. A report from Android Police says the device still hasn?t received a security update and given the coinciding time it might never get another one.
The Google Pixel C does not appear on the update schedule help page, but it disappeared a long time ago. The Nexus 5X and Nexus 6P also have stopped receiving security packages, although they were one month over the 18-month schedule.
Back when the Pixel C was announced, it had an impressive 2560 x 1800 resolution and a quad-core CPU at 1.9 GHz. Initially launched with Android 6.0.1 Marshmallow, it was eventually got Android Oreo but not Pie.