\sout defined by \usepackage{ulem} (doc) is designed for text mode. While it seems to be allowed in math mode it doesn't actually do the right thing. For example, in LuaLaTeX:

• \text{\sout{ab}} works fine
• \text{\sout{$ab$}} works fine
• $\sout{ab}$ compiles but shows an underline, which is not the intended use of \sout

KaTeX seems to do the opposite:

• In text mode, it incorrectly gives: "KaTeX parse error: Can't use function '\sout' in text mode"
• In math mode, it behaves like a proper strike-out, even though it doesn't seem intended to be used as such with ulem's current behavior.

Screenshot of LuaLaTeX behavior for the following LaTeX

a\sout{bc}d

a\sout{$bc$}d

$a\sout{bc}d$