Edit: getting downvoted without any reasoning provided, so I assume I'm incorrect, there are more/less ways of storing data in the future for Safari users?
Cookies can either be set in HTTP responses or through the document.cookie API, the latter sometimes referred to as client-side cookies. With ITP 2.1, all persistent client-side cookies, i.e. persistent cookies created through document.cookie, are capped to a seven day expiry.
Indexed DB, LocalStorage, Media keys, SessionStorage, Service Worker registrations
Since cookies are not mentioned, I'm assuming it's NOT affected by the 7 day cap but will instead continue to work as normal (except for the fact that 3rd party cookies will stop working, which is a Good Thing)
So in order to have a long-lived cookie, you essentially need to treat them as read-only client side, and push any and all update/write logic to the server such that it'll return a set-cookie header with any changes you require.