I don't think we should. There's a few reasons.
We're still in a mess with designer reasons: lots of people are over-sensitive to determining a question is designer reasons, but really the only thing that wasn't working is “explain to me their thought process behind why they did that.” The mod team has had to reopen a lot of questions incorrectly caught under a designer reasons crosshair. I'd rather this be carefully filtered after posting.
Second, designer reasons only demonstrably failed for D&D and similar. There's some history for designer reasons questions working fine for Fate, but those work specifically because the authors write in enormous volumes and detail about their thought processes and philosophies, often within the pages of the very same books, or within their personal blogs. Other games are yet unknown on that front. D&D doesn't tend to have anything like this happening except on very rare occasions. This has made me disinclined to make blanket statements about designer reasons never being on topic for any system or question, just treating things as they come.
Third, we don't list everything we do/don't accept in the tour. It isn't exhaustive, it just hits the most common/important things. We have multiple different help pages and topics covered in questions to avoid asking and the meta. We ought to handle things case by case and let people know what's off topic.