Crushed cinder block a substitute for crushed stone?
Depends on the height of the retaining wall and what it’s holding back.
We like footings on undisturbed soil. In fact, depending on the soil we might compact it too.
If the soil is soft, we may require a few feet be removed and replaced with compacted gravel. This will help distribute the load over a greater area (the gravel area is usually 2 times wider than the footing) and rests on soil several feet lower so it will be stronger.
However, the gravel is not a random mix. The gravel is uniformly mixed with specific set of type, grade, amount, etc. that is designed to be compacted. So, if you just break up the cinder blocks, the breakage will be random and cannot be compacted properly.
If the retaining wall is only 2-3’ tall, then this will not be a problem. However, if the retaining wall is 8’ or more, then the compacted gravel must be done properly, including doing the lifts in 6-8” each and compacted as it’s installed up to 95% by volume.