If it’s like Matomo’s opt-out system, that cookie is required – it’s the very thing that turns on the opt-out.
When you opt-out of Matomo, the message warns that “if you clear your cookies, delete the opt-out cookie, or if you change computers or Web browsers, you will need to perform the opt-out procedure again”.
Matomo also honours the browser’s DNT setting, which is a) nice and b) rare.