The answer is 'TC39' in 'Ecma International'. Let explain this answer in more detail.
First, let's find out what 'Ecma International' is.
According to [the homepage of Ecma International > About Ecma > Mission], it creates standards about Information and Communications Technology (ICT) and Consumer Electronics (CE).
According to [the homepage of Ecma International > Committees], there are many divisions. The divisions are called 'Technical Committees (TC)' and 'Task Groups (TG)'. Task Groups (TG) are subdivisions of Technical Committees (TC).
Also, in the page, we can find out a Technical Committee (TC) that covers ECMAScript is TC39. You can go to the page of TC39 by [TC39 ECMAScript > click 'ECMAScript'].
Where Can I Find the Documents?
There are two ways to do this.
Go to the page of TC39 as I mentioned above. And then go to [Published Standards]. Click the standard you want to see. I think the main standard is 'ECMA-262'.
Go to MDN and search what you want. Then, there will be a link which connects to the part of the document which covers it.
Where Can I Find the Proposals?
One of the reasons I am writing this article is to let you know the way to find out how ECMAScript will be changed in the future.
First, we should know how proposals are processed. If you go to [the page of TC39 > Activities > TC39 development process], you can find out how they are processed. There are five stages: [strawman (stage 0) -> proposal (stage 1) -> draft (stage 2) -> candidate (stage 3) -> finished (stage 4)]. Proposals should pass each stage to be accepted. Of course, the higher the stage level is, it is more likely to be accepted.
Then, Let's find out the proposals. There are two ways to do this.
Go to the GitHub page of TC39. Then, go to the 'proposals' repository. There, you can find out each stage's proposals.
Go to MDN and search what you want. Then, there will be a link which connects to the proposal of it.