It is not.

With the exception of the "Full Flex" fares (Y, B, M) every fare is available as a branded fare (Light, Standard, Flex).

There is, unfortunately, no way on the website to book a specific fare bucket, only the branding can be selected. If you want to book a Y fare, you must call or use a travel agent. Expect to pay through the nose however; they are much more expensive than the "Flex" branded fares which have a standard upcharge from the base fare.