Older SCP articles make mention of D-class personnel being terminated on a monthly basis; whenever 30 days have passed, all D-class detained at Foundation sites are killed. However, such a practice raises far too many questions, both in and out of universe. Eliminating all D-class personnel every month, aside from being a completely pointless act of cruelty, would be totally counter-productive as D-class are an expensively finite resource, and would force the Foundation to acquire even more prison convicts to replace their dead predecessors.
At least according to the file for SCP-2193, the so-called "monthly termination" of D-class is actually a memetic infohazard anomaly affecting SCP documents; which compels staff members who read about it to round up all D-class personnel at their site, unwittingly sacrificing them to an unknown entity which causes them to float up high into the sky and disappear (presumably leaving Earth's orbit for outer space). Affected Foundation personnel during these events will falsely remember that the missing D-class victims were simply terminated with firearms by the security guards.
Made one :)