Yes, I was not strictly correct, it is a feature of AV1, but Netflix played an active role in its development, in rolling out the first implementation, and in AV1 codec development overall.

Prior codecs all had film grain synthesis too (at least back to H264), but nobody used it. Partly because it was obviously artificial and partly because it was too expensive to render, since you had to copy each frame to apply effects to it.

AFAIK, FGS support is the exception instead of the rule.

h.264 only had FGS as an afterthought, introduced years after the spec was ratified. No wonder it wasn’t widely adopted.

VP9, h.265 and h.266 don’t have FGS.

Did they remove it from the specs? Wouldn't surprise me. But since it's just extra metadata and doesn't affect encoding you could still use the old spec.

I don’t think it was ever part of the spec. It would surprise me if it was, because once a spec has been finalized and voted on, changing it is complicated. Getting agreement the first time is already difficult enough.