I'd say that if the user is using a private browsing mode, they are explicitly indicating to the browser not to store anything, and they will know that will hap

I'd say that if the user is using a private browsing mode, they are explicitly indicating to the browser not to store anything, and they will know that will happen. It's the whole purpose of the feature, so I don't think it counts as "silently" doing anything. I don't think there's any reason to then potentially break web pages that (quite reasonably) expect storage APIs to work.