"the cookie is strictly necessary to provide an ‘information society service’ (eg a service over the internet) requested by the subscriber or user. Note that it must be essential to fulfil their request – cookies that are helpful or convenient but not essential, or that are only essential for your own purposes, will still require consent."

If the site's using a common framework, convenient and essential uses will be combined in a "session id" cookie which needs re-developing, or just slap a cookie warning on it.

https://ico.org.uk/for-organisations/guide-to-pecr/cookies-a...