Whether or not you declare it essential is irrelevant. What matters is if it actually
is essential.
As for localstorage, read the guidance from the ICO:
https://www.ico.gov.uk/~/media/documents/library/Privacy_and...
Third paragraph: "These changes apply to storage or gaining access to information stored, in the device of a subscriber or user. This means the use of cookies and similar technologies for storing information."
Fifth paragraph: "The Regulations also apply to similar technologies for storing information. This could include, for example, Locally Stored Objects (commonly referred to as "Flash Cookies")."