I'm a believer that the challenge's OP shouldn't have the final say as to the challenge anyway (and our previous discussion on the issue agrees with that). If there was any way to release all my questions to the community in the hope that they'd make them better, I'd do that. I'd hoped that deleting my account would make it clear that I didn't want ownership over the things I'd posted, but apparently not.
In general, I think the ideal outcome would be for Meta consensus on, e.g., allowed I/O formats to be clear enough that there's only one standard way to write a challenge anyway. Having the OP decide separately for each challenge simply leads to inconsistency and some of the challenges being suboptimal (in addition to being a lot of work for the collected OPs). This implies an answer to your actual question: ask here on Meta, ideally aiming to get a precedent that can be used for a range of challenges. (As it happens, the challenge you linked explicitly attempts to defer to the Meta consensus, but we currently only have a Meta consensus for I/O methods, not I/O formats.)
(By the way, you shouldn't put much stock into challenges being highly upvoted, especially with this challenge in particular. I knew even before I'd posted it that it was inevitably going to be voted into the stratosphere simply because the ratio between how easy it is to answer, and how impressive answers are to someone who's unfamiliar with golfing, was likely to pin it to Hot Network Questions for several days and attract a large number of drive-by upvotes as a result. Because the votes were inevitable, being highly voted thus doesn't imply much about the quality of the challenge.)