Fannie Mae and Freddie Mac as substitute benchmark bonds
Fannie Mae and Freddie Mac bonds have long been viewed as having an implicit government guarantee, and though they received government support during the crisis, they never missed a payment to bondholders. This implicit guarantee was strengthened in July 2008 when Congress passed the Housing and Economic Recovery Act of 2008, which created a channel through which the Director of the Federal Housing Finance Agency could place them in conservatorship, which was done in September 2008. The GSEs remain in conservatorship today, and have access to lines of credit with the US Treasury. So while it's true that they still do not have an explicit guarantee today, it's easy to see why they're viewed as having a guarantee, at least implicitly.