You have many notable query plans?

I the value of 6 for the retention time is correct, MDW is finding over 100000 notable query plans per day. That is a lot, and it makes me suspect that you have a bad application that inlines parameter values into SQL string.

But it could also be the case the expiration is not working. I don't have the MDW installed, so I don't know exactly what's in snapshots.notable_query_plan. But if there is a date column, you should be able to tell whether expiration is working or not.

Exipration plan does not seems to be issue here. I have checked only 206 expired orphan plans exists.

Used below query to find orphan records - https://support.microsoft.com/en-us/kb/970014

SELECT COUNT(*)
FROM snapshots.notable_query_plan AS qp 
        WHERE NOT EXISTS (
            SELECT * 
            FROM snapshots.query_stats AS qs
            WHERE qs.[sql_handle] = qp.[sql_handle] AND qs.plan_handle = qp.plan_handle 
                AND qs.plan_generation_num = qp.plan_generation_num 
                AND qs.statement_start_offset = qp.statement_start_offset 
                AND qs.statement_end_offset = qp.statement_end_offset 
                AND qs.creation_time = qp.creation_time);