The Nazis also ran work camps which ultimately did mass extermination at a slower simply by virtue of workload and food input. High and low temps also work as well as Poor health and sanitation.
Easy enough to construct a circumstance where every inmate dies in one to ten years and cheaper the boot.
Why would they keep all of their own prisoners alive in the first place knowing that anyone who would be released is either actually dangerous or an innocent who now hates the state.
It would save money to continually allow the weakest to die from disease privation templature. A slow motion Holocaust where there isnt any smoking guns of gas chambers where the crematorium fires are always burning but the cause of death is varied