It is a terrible, sad shame that other developers abusing a very common task has scared everyone away from using an API just because of the "callback" term. APIs are quite common in modern technologies. Upgrading a theme simply cannot be done with one-click installs without doing a callback, not for premium add-ons and themes at least.I would like to be able to continue to upgrade manually and to deactivate the callback.
Is this possible?
If we cannot check what get's installed at our server prior to install it, we cannot use your style any more. Which would be very sad.
That said, we knew this would be a concern for some. Yes, by not entering an API key there will be no callback. The theme files alone will be available in your orders page by viewing the changelog. The only drawback is no automatic alert letting you know there is an upgrade and no easy install/upgrade of our products. This extends to branding removal and the like.
TLDR; Callbacks are entirely optional.