In il2code.tex there is such text:
%% Alternative \hyphenchar ("je-li" is no "je\hyphenchar li").
\chardef\extrahyphenchar=156
\def\extrahyphens{%
\hyphenchar\tenrm=\extrahyphenchar
\hyphenchar\tenbf=\extrahyphenchar
\hyphenchar\tentt=\extrahyphenchar
\hyphenchar\tensl=\extrahyphenchar
\hyphenchar\tenit=\extrahyphenchar
\defaulthyphenchar=\extrahyphenchar}
What is the benefit of this?

\exhyphenchar(explicitly specified hyphen char) and\prehyphenchar,\preexhyphencharetc., so setting\hyphencharis almost no-op in LuaTeX. – user202729 Jul 02 '22 at 15:04