As a native Shanghainese speaker I am extremely confused (and curious!). Cantonese at least has a written form, but Shanghainese AFAIK has no written form, so what does "Shanghainese dialectal spelling" even mean? :\
The fact you don't know about it doesn't mean it doesn't exist. Same goes for Taiwanese Southern Min: a lot of natives think "it's not a written language" where in fact there are both romanizations and a way to write it with Chinese characters.
The central government is actively fighting standardization effort (both romanization and writing in Chinese characters) and push the fiction of Wu and other Sinitic languages (Cantonese, Hakka, Min, etc.) as merely dialects, as a way to destroy them. The method is not new, is has been policy in France, Taiwan and probably other countries. It's a real shame that Apple is validation China's propaganda on that front.