Per a recent update to the Encoding Standard, encoding to UTF-16 is no longer supported by default.

Statistically speaking, there were no users of those encodings with the native implementations of the API in Chrome. There was also no other place where the Web required UTF-16 encoding - even pages served as UTF-16 post form data back as UTF-8, unlike other encodings.

Since this is breaking change for the polyfill, the minor release version was bumped. You can continue to use the NONSTANDARD_allowLegacyEncoding option and pass one of the UTF-16 labels (utf-16le, utf-16be, etc) to use these encodings.