thanks for clarification.
Kind of confusing, since your product description says "Includes all features of the Pro plan, our most powerful plan"
Is it possible to stack another code in future as well, for existing AppSumo customers, even though, when you don't provide it on AppSumo anymore?

I understand what you mean, the deal here on AppSumo is the equivalent of our Pro plan, but with a stackable storage limit. Right now, you can purchase codes to stack 5GB with each code.We will always provide a path forward for lifetime users, but I don't know what that would look like, if it's offering the option to stack more 5GB coupons or something else. But you will have always have a way to keep using the platform in the long term, when your course business starts growing.

so you will keep the option to buy another 5 gb for 99,- one time payment in future? As it is now available within admin-section?

For the foreseable future, all options on the admin dashboard for upgrading lifetime storage will remain.