\documentclass[12pt]{article}

\usepackage{fontspec}
\usepackage{polyglossia}

\usepackage{makeidx}
\makeindex

\setdefaultlanguage{korean}
\setmainfont{Noto Serif CJK KR}[Language=Korean,Script=Hangul]
\setsansfont{Noto Serif CJK KR}[Language=Korean,Script=Hangul]

\pagestyle{empty}
\begin{document}
\thispagestyle{empty}

ᄉᆞ\index{ᄉᆞ (archaic)}.
ᄊᆞ\index{ᄊᆞ (archaic)}.
ᄊᆞ〯\index{ᄊᆞ〯 (archaic with tone mark)}.
%\index{ (Hanyang PUA)@ᄉᆞ (Hanyang PUA)}.
\index{ (Hanyang PUA)@ᄊᆞ (Hanyang PUA)}.

쓰\index{쓰 (composed)}.
쓰\index{쓰 (decomposed)}.
스\index{스 (composed)}.
%스\index{스 (조합형)}.

ᄴᅳ\index{ᄴᅳ (archaic)}.
\index{ (Hanyang PUA)@ᄴᅳ (Hanyang PUA)}.
%ᄼᅳ\index{ᄼᅳ (archaic)}.
ᄽᅳ\index{ᄽᅳ (archaic)}.
%ᄾᅳ\index{ᄾᅳ (archaic)}.
ᄿᅳ\index{ᄿᅳ (archaic)}.
ᅀᅳ\index{ᅀᅳ (archaic)}.

\printindex

\end{document}


\index{}.