https://github.com/cri-o/cri-o/blob/main/docs/crio.conf.5.md#crioruntimeruntimes-table says:

The "crio.runtime.runtimes" table defines a list of OCI compatible runtimes. The runtime to use is picked based on the runtime_handler provided by the CRI. If no runtime_handler is provided, the runtime will be picked based on the level of trust of the workload.

Since runtime_handler is not an option/parameter, the underscore shall not be used. Instead the spelling shall be “runtime handler”.