Ah my bad, I think that's the wrong link describing the old process. I remember a recent talk by Axel (author of that post) describing the new process where features are accepted as soon as they are done (i.e. stage 4). But I can't find any articles by him to back this up, so maybe my memory is failing me ;-)

Update: ok, as soon as a proposal reaches stage 4 it's considered finalized and part of the standard. A proposal can reach stage 4 at any time during the year. At that point browser vendors will start implementing those. https://flaviocopes.com/es2019/