No, endianness is not a feature of UTF-8 encoding. There isn't a UTF-8LE and a UTF-8BE. That's because the codeunit is bytes.

Forget "decoding", you have to parse JSON. But you don't have to figure out how it's encoded first. Because it's a byte format. You already know.

There isn’t a UTF-8LE/BE because it is implicitly BE for wide characters. Any byte in a WC sequence cannot meaningfully be interpreted (exc character class, page etc) without its companions, so not just bytes. There is an element of presentation that must happen before “mere bytes” are eligible for JSON