The summary of the issue is clear enough, but the description is mangled. WorkspaceAction.getLabels() is clearly documented to be the labels of the node owning the workspace, and the implementation matches the documentation and will not be changed.
What you are asking for is access to the ExecutorStep.getLabel()—the requested label expression. This is not really the responsibility of WorkspaceAction, which is used in a couple of other places, of which WorkspaceStepExecution has no associated label expression, and CpsScmFlowDefinition could (but currently does not, and anyway that information would be statically available in the job definition).
What might be more generally useful is to record the step configuration as an action in the StepNode.
JENKINS-31582 requests that the plugins associated with the step be recorded, but we might as well just record the whole configuration, since it is typically not very large, and the plugin information can be reconstructed from that.