This attribute is an implementation detail of the CLDR Japanese tailoring, a special hack to provide the fourth-level Hiragana/Katakana distinction of JIS X 4061. The attribute is only meaningful when the tailoring erases all higher-level distinctions between Hiragana and Katakana, and it is required for correct Japanese sorting using the CLDR Japanese tailoring. Users should never change any tailoring’s value for this attribute, and implementations should be free to implement the Hiragana/Katakana distinction of JIS X 4061 by other means (e.g., a true quaternary tailoring).
The LDML <setting> attribute hiraganaQuaternary needs to be retained, for use in the CLDR Japanese tailoring, until CLDR replaces this hack with a different mechanism.
Look at this for 23; want to have implementation in place.
DTD change (deprecating the hiraganaQuaternary attribute), should do this in CLDR 25; ICU should by then also support true quaternary relations.
Milestone 25rc deleted