ISO 8601 is large standard and predates the internet (as we know it) and seems like an attempt of formalizing all the many different ways people expressed dates (on physical papers in addition to computer systems), and also some ways no-one would ever use.

I think you’ll find a lot of APIs (or formats) claiming to support an “ISO 8601 date” when in fact they mean a date of the form “YYYY-MM-DD” — it’s a common misunderstanding, but I think better that, than actually allowing the full range of dates allowed by the actual ISO 8601 standard :)