Idk that I'd call it clearly defined minimums/restrictions. It might just be that 20" tends to be the shortest order they commonly get and therefore configured the slider that way, or that they didn't anticipate the market for shorter cables for split keyboards. I sent an email to find out, because I don't think it's all that unrealistic to think that if you inquire a custom cable company might be willing to cut a shorter piece of cable. It's not like I demanded they make it.

I'm very sympathetic to people doing their jobs, and have worked in customer support/client facing roles most of my career. But not responding to all the questions in a customer's email, especially when there are only two of them and they're related, and then again not reading/looking into the issue before responding is bad customer service.

Also, thanks for the recommendations on where to find other vendors.