The color() function used to have a fallback, which added significant complication to implementation, bulked up the object model, without adding much value, and was dropped.

The device-cmyk() function still as the same style of fallback.

Is this needed? The same arguments against the color() fallback apply here. On the other hand I'm aware the primary use is CSS to PDF converters (which typically do not construct object models) and maybe they rely on having a fallback? What do the proprietary cmyk extensions in various PDF generators use, do they have a fallback or do they depend on the usual CSS falback mechanisms?

Note that now that the preferred way to implement device-cmyk() is to associate it with an ICC profile, so generating an RGB preview for example is much more tractable than it used to be.

Unless there is significant feedback in favor of keeping the fallback, I propose to remove it.