For those of us using a customized theme, especially right after a big migration from another platform, it would be very useful to be able to test all theme customizations before actually publishing them. Right now, the only way to do that is by using the preview of unpublished customized themes, but that preview has a lot of limitations, and, more often than not, it does not reflect the actual end result.
A Guide sandbox would also be very useful for testing restructuring options, too. For example, we're currently preparing a major restructuring of our Help Center (new categories, new subsections, breaking down existing articles to smaller ones). In order to have the least impact on our users (who span different time zones) we should test and deploy outside work hours (way outside, in fact) and we would still run a significant amount of risk.
Thank you for taking the time to provide us with your feedback. As James mentioned this feature is available but for our Enterprise plans. Please feel free to add further feedback on this feature area as you wish. Thank you again!