The TextEncoder API never supported legacy encodings (such as 'shift_jis', 'windows-1252', etc) except for two UTF-16 variants ('utf-16', 'utf-16be'). Usage was minimal and support was removed from the spec. The TextEncoder constructor will no longer take an argument (if one is passed it is ignored, as is the standard for DOM APIs), and will always encode to "the encoding" (utf-8).
Docs: https://developer.mozilla.org/en-US/docs/Web/API/TextEncoder (Docs still refer to the utfLabel parameter, which is no longer supported.)