What if I purchased iCloud backup then i stopped subscription.what will happen to my stored data

You do not lose any data but since your free 5GB allowance is full, going forward you may get alerts every few seconds that your iCloud is full, any new data, contacts, photos, calendar will not sync to the cloud and all your iCloud emails will bounce back to the sender. Your account becomes useless until you delete down to 5GB of data you have on iCloud or pay for the extra storage.

Trust me, if you discontinue a subscription and go back to just the free 5GB, after 30 days you WILL lose data. Apple will purge the excess to get your storage down to the free 5GB limit. They don’t just hang on to your unpaid excess storage content forever.

After 30 days grace, anything beyond 5GB will be purged. They give you 30 days to move whatever you want to keep to some other storage media.