The Lockheed SR-71 and the (old) Lockheed U-2 were engineered to maintain cabin altitudes in excess of 25000 feet AMSL at an altitude of more than 70000 feet AMSL.
However, in both airplanes, pilots are required to wear space suits which maintain the safe pressurisation and oxygen content anyway. If the space suit develops a pressurisation leak, I think it's unlikely that the very high cabin altitude in these airplanes would buy the pilots any more time to descent to a safe altitude safely as the time of useful consciousness at 25000 feet AMSL is the order of a few minutes. Furthermore, it seems strange that aerospace engineers would install a pressurisation system on the Lockheed U2, an airplane already struggling to be as light as possible.
So why is this pressurisation maintained at such a high altitude in the airplanes?