It was completely optional for websites to support this. A few did at first.
A lot of people internally wanted it to be on-by-default, but the argument was that if it was, nobody would respect it – after all, what tracking platform would willingly only track the 0.1% of people who went into the settings to enable it? (Internet Explorer did eventually enable it by default, which got them good press but ultimately killed the feature since everyone stopped respecting it.)
Overall, I'm happy to see this sunsetted. I don't think it actually did anything – in fact, I think it implies that it did way more than it did, so it was just a faux feeling of security.
(All that being said, I would love if the cookie modals on each site became browser-level, but I'm sure there's many reasons that hasn't happened yet. And I suspect a bit reason starts with a G and ends with an Oogle.)