Rationale (August, 2010):

CWG felt that an attribute was a more appropriate representation for this feature.

C has rejected the notion of attributes, and introduced the noreturn facility as a keyword. To continue writing clean, portable code we should replace the [[noreturn]] attribute with a noreturn keyword, following the usual convention that while C obfuscates new keywords with _Capital and adds a macro to map to the comfortable spelling, C++ simply adopts the all-lowercase spelling.