How can we possibly tell that there is a possibility that a wallrt service has the info of our seed phrases on their database? Is it because of they being open-source/close-source? Does being closed means that they're the only one that fully knows what the codes are so they can hide something?
If the wallet is close-source, there is no way to know that. They may claim that they have no access to users keys, but there's no guarantee that they tell the truth.
If the wallet is open-source, one can read the code and know whether the keys are stored on their server or not.
