I'm looking at the JSON5 spec and it appears it does not introduce a capital \U escape sequence for Unicode characters outside the Basic Multilingual Plane (BMP). It's not brought up often, but in JSON you do need UTF-16 surrogates to write an escape sequence for Unicode characters outside the BMP. Consider the Hamburger Emoji (U+1F354). Instead of escaping it as "\U0001F354", you need to escape it with UTF-16 surrogates "\uD83C\uDF54". This is both cumbersome for humans and not in accordance with the Unicode Standard [1]. It's ironic, but many (most?) of the "JSON for Humans" flavors of JSON tend to overlook this.

[1] See Chapter 3.8 "Surrogates" of the Unicode Standard.