A keystone-less arch.
People who explain arches to other people often like to point out how the keystone is the most important part of an arch. It's isn't . Actually it isn't any more important than any of the other voussoirs that make up an arch. It's just that its often the last one to go in.
Removing it is no more likely to have the arch fall down than removing any of the other stones.
There are many arches that don't even have keystones.