为什么规范中没有将 `let` 指定为保留关键字

我本来期待找到两者const and let作为保留关键字。我只找到了 https://www.ecma-international.org/ecma-262/8.0/index.html#prod-ReservedWord const，但是没有let:

Keyword::
    await
    break
    case catch class const continue   <---- `const` here
    debugger default delete do
    else export extends
    finally for function
    if import in instanceof
    new
    return
    superswitch
    this throw try typeof
    var void
    while with
    yield

Is let一个关键字？如果没有，为什么？

let wasn’t reserved before ES6 ES5 strict mode https://es5.github.io/#C, so it can’t be treated the same as the rest of the reserved words for backwards-compatibility reasons like this:

var let = 5;

您会在新构造的规范中发现很多例外情况（例如let let = 5;是无效的）。