Need to now any appian feature which give us details of unused variable in process model , anything system driven ?

I would just generate the documentation (Tools/Generate Documentation) for the process model and then search for the process variable on that documentation page (Ctrl+f on Windows). If it is only found once it would suggest it is defined but unused, if not you can see where it is referenced.

Ideally if you are breaking your processes down into sub-processes to reduce complexity, enable better memory handling, etc., then you shouldn't really have too many process variables defined in any one process model so identifying unused ones shouldn't be too difficult.

I couldn't found place where it is mentioned that this Process Variable is unused or not useful.

I need that details only as need to analyze all PM where unused variables are occupying unnecessary memory space.

As I said, if you generate the documentation for your process model and then use the 'Find' option on your browser and search for a particular process variable by its name you should be able to see every time that process variable is referenced by your process model. Every process variable will be found at least once as it will appear in the Process Variables table, if that is the only place it appears then it is not used by that process model so it may be safe to remove it. You will need to check if that process model is called from anywhere else, either as a sub-process, from the Start Process smart service, as a related action, etc., as they may also need to be updated if the process variable was also a parameter (use Dependents to check that).

Unfortunately this approach is a manual approach, one process model at a time and one process variable at a time but if developers have been following good practice they should only be creating process variables when they are needed and removing any that are unnecessary when the process model is refactored.

If you wanted a more automated approach I suspect you would have to export the process models in a patch file and then try to interrogate the xml files contained within that patch file to identify the process variables and to search for any additional references to them.

As I said, if you generate the documentation for your process model and then use the 'Find' option on your browser and search for a particular process variable by its name you should be able to see every time that process variable is referenced by your process model. Every process variable will be found at least once as it will appear in the Process Variables table, if that is the only place it appears then it is not used by that process model so it may be safe to remove it. You will need to check if that process model is called from anywhere else, either as a sub-process, from the Start Process smart service, as a related action, etc., as they may also need to be updated if the process variable was also a parameter (use Dependents to check that).

Unfortunately this approach is a manual approach, one process model at a time and one process variable at a time but if developers have been following good practice they should only be creating process variables when they are needed and removing any that are unnecessary when the process model is refactored.

If you wanted a more automated approach I suspect you would have to export the process models in a patch file and then try to interrogate the xml files contained within that patch file to identify the process variables and to search for any additional references to them.