> It is so elegant and common that even though it is not part of a standard ...

YYYY-MM-DD hh:mm:ss seems valid ISO 8601 to me, isn't it? Neither the "T" nor the timezone are required as far as I recall.

EDIT: The site says ISO 8601-1:2019 requires a T for datetimes, and that even though previous editions allowed for no T, a space was never allowed. This is shocking news to me.