Process automation Task data association and Retrieve Task REST API

I do not find the key identifier field “exec.keyIdentifier” which was existing before (In gen2) to retrieve/update any running task.

In Gen2 we used to have this field to retrieve the task and update task with the payload using Factory REST API, but in Gen3 we do not have this field. Is there any alternative field created in Gen3 to meet this need, If so please provide the details or provide any workaround.

Not available in GEN3 but was available in Gen2: