Relational override symbol

Package stix has it:

\documentclass{article}
\pagestyle{empty}
\usepackage{stix}
\begin{document}
$a\dsub b$
\end{document}

Actually texdoc comprehensive is a very good source for information like this.

You're lucky: the minus sign and \vartriangleleft have the same width.

\documentclass{article}
\usepackage{amssymb}

\makeatletter
\DeclareRobustCommand{\reloverrideleft}{\mathrel{\mathpalette\rel@override\vartriangleleft}}
\DeclareRobustCommand{\reloverrideright}{\mathrel{\mathpalette\rel@override\vartriangleright}}

\newcommand{\rel@override}[2]{%
  \vphantom{#2}%
  \ooalign{$\m@th#1-$\cr$\m@th#1#2$}%
}
\makeatother

\begin{document}

$A \reloverrideleft B_{\reloverrideleft}$

$A \reloverrideright B_{\reloverrideright}$

\end{document}

With other math fonts it may not work flawlessly, I'm afraid.

For acmart we need to clip the minus sign:

\documentclass{acmart}
\usepackage{trimclip}

\makeatletter
\DeclareRobustCommand{\reloverrideleft}{\mathrel{\mathpalette\rel@override\vartriangleleft}}
\DeclareRobustCommand{\reloverrideright}{\mathrel{\mathpalette\rel@override\vartriangleright}}

\newcommand{\rel@override}[2]{%
  \vphantom{#2}%
  \ooalign{%
    \hidewidth\clipbox*{{0.17\width} 0 {0.83\width} {\height}}{$\m@th#1-$}\hidewidth\cr
    $\m@th#1#2$\cr
  }%
}
\makeatother

\begin{document}

$A \reloverrideleft B_{\reloverrideleft}$

$A \reloverrideright B_{\reloverrideright}$

\end{document}