It doesn't involve any loss of data, but it could degrade your data safety. If you cancel your subscription, your quota is reduced and, if you're now over quota

It doesn't involve any loss of data, but it could degrade your data safety. If you cancel your subscription, your quota is reduced and, if you're now over quota your desktop client stops syncing. It may also prevent Camera Uploads from working on your phone. So you might delete a photo from your phone assuming it was uploaded to Dropbox but it wasn't because you were over quota.