G4StepStatus.hh defines the step status fUserDefinedLimit. It appears this step status is never set. When a forced process PostStepDoIt method is called because the step length exceeded that limit set with G4UserLimits, the postStepPoint step status is not set to 'fUserDefinedLimit' but rather set to 'fPostStepDoItProc'. I have not been able to find any combination of processes/force conditions that will produce the 'fUserDefinedLimit' step status.
This is the obsolete option and not available to use now.