Command to monitor Jenkins idle status returns false if even 1 executor is being used

Using Jenkins LTS 2.421
Jenkins Nodes on Java 11

We use this to alert us if nodes are being used 100% and jobs start queuing.

Expected behavior:
When running the following command it should return idle : true, if there are any idle executors.
curl -X POST -L --user username:token | grep “idle”

I noticed that both Linux and Windows nodes are returning false if even a single executor is currently being used. If no executors are being used it does return true.

Is there another way to accomplish this task if this solution is no longer viable.

Maybe this is a bug or maybe a better method may be used?