> In most cases, the add-on just blocks or hides cookie related pop-ups. When it's needed for the website to work properly, it will automatically accept the cookie policy for you (sometimes it will accept all and sometimes only necessary cookie categories, depending on what's easier to do). It doesn't delete cookies.
Well, no, because there are non-cookie mechanisms of tracking to which you'd want to refuse permission, and because for sites where you do want to have some cookies (e.g. to log in to an account) you'd want to refuse permission to use that information for other purposes.
You have no way of proving whether any cookie or non-cookie mechanism is employed for a purpose you did not give permission for.
What you can do is eg. not enable the microphone when the website asks for it, not send the GA cookie back with requests to spin the visitor counter, or make the browser pretend that you have a bog-standard screen resolution and font selection. They will not have the information, so only the lack of information can be used.
If the cookie that stores your logged-in status is used for other purposes like getting more relevant ads in front of your eyeballs, that sucks. One can only hope that they are separated by functionality, or the candidates for more dubious activities are given out by a third party.
The main limitations on companies are legal, not technical, imposing costs on them that make it not worth to break the law.
There are obvious ways of proving whether any cookie or non-cookie mechanism is employed for a purpose you did not give permission for, namely, audits of their systems and testimonies of their employees, which has resulted in quite a few huge fines being assessed and the illegal activity stopped, and will result in more.
We simply have to not legitimize this being done as "business as normal" and have to make it clear that they are not permitted to do so - all the really big impact comes from the large megacorps who eventually have to stay above the board legally.
