How To Include a Reversed Angle Symbol (⦣) in LaTeX

You can use reflectbox:

\documentclass{article}

\usepackage{graphicx}

\providecommand*{\revangle}{\reflectbox{$\angle$}}

\begin{document}

$a \angle b$

$a \revangle b$

\end{document}

You can use the symbol in latex if you have a font for it, for example if you are using unicode-math it is available as ⦣ or \revangle and if you are using pdflatex you could use the stix package which again uses the commandname \revangle

with lualatex or xelatex :

\documentclass{article}

\usepackage{unicode-math}
\setmathfont{Stix Two Math}
\begin{document}

$a ⦣ b$

\end{document}