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.
{ "Domain": ".fullstory.com", "Name": "fs_optout", "Value": 1, "Path": "/", "Expires": null, "Store ID": "firefox-default", "First Party Domain": null, "Secure": false, "Session": true, "Http Only": false, "Host Only": false }