The Secret Space is a private space on your device. It is encrypted and stored in the storage dedicated to the Piktures app. It is therefore secure from prying eyes.
If you get your phone lost, or stolen, there is noway from someone else to get access to your private photos.
But it is not backed up online. It remains on your phone, and the Piktures team does not have any way to access it. There is also no way for the team to restore your files, as we don't have any way to do so.
Export your Secret Space before deleting Piktures
If you want to delete Piktures, you need to take steps to save your Secret Space, in order not to lose those precious photos.