> An Options object is a worse API, since it assumes an implicit dependency.

No, it would be an explicit dependency.

    // definition:
    function decodeText(input, [format | options])

    // use:
    decodeText(someBuffer, 'utf8')

    // or:
    const opts = {encoding: 'utf8', fatal: true, ignoreBOM: false}
    decodeText(someBuffer, opts)
    decodeText(somethingElse, opts)

> Why?

Because a pure function is idiomatic, concise and clear. Its easier to use, easier to document and easier to understand. The only benefit of a class is that it encapsulates its state behind an interface. That makes sense for a BTree or a database. But TextDecoder has no mutable state. It has no state at all except for its passed-in configuration.

Decoding text is a verb not a noun. In programming, verbs are translated to functions, not classes. You don't have an "adder object" to add things. You don't have to instantiate a JSON stringifier before calling JSON.stringify. If thats not obvious, you may have spent too long in OO languages.