Il suffit de réinsérer le même code dans un environnement *scope* avec l'option `xscale=-1`
\documentclass[12pt, a4paper, english, BCOR = 0pt, DIV = 22]{scrartcl}
\usepackage[T1]{fontenc}
\usepackage[light, largesmallcaps]{kpfonts}
\usepackage{tikz}
\usepackage{float}
\usepackage{babel}
\begin{document}
\begin{figure}[H]
\centering
\begin{tikzpicture}[scale = 1.2]
\tikzstyle{N}= [circle, minimum size = 2 pt, inner sep = 2 pt, draw = black, fill = yellow]
\draw [line width = 3 pt, color = red, rounded corners] (89.8:5.7) -- (89.8:5.5)
arc [start angle = 89.8, end angle = 83, radius = 5.5] -- (83:5.7)
arc [start angle = 83, end angle = 75, radius = 5.7] -- (70:7)
arc [start angle = 70, end angle = 66.7, radius = 7]
-- (65.5:7.3) ;
\begin{scope}[xscale=-1]
\draw [line width = 3 pt, color = green, rounded corners] (89.8:5.7) -- (89.8:5.5)
arc [start angle = 89.8, end angle = 83, radius = 5.5] -- (83:5.7)
arc [start angle = 83, end angle = 75, radius = 5.7] -- (70:7)
arc [start angle = 70, end angle = 66.7, radius = 7]
-- (65.5:7.3) ;
\end{scope}
\end{tikzpicture}
\caption{Une ligne rouge}
rouge et la même en vert (mais à l'envers)}
\end{figure}
\end{document}
Il est possible aussi de passer l'option `xscale=-1` à la commande `\draw`
\documentclass[12pt, a4paper, english, BCOR = 0pt, DIV = 22]{scrartcl}
\usepackage[T1]{fontenc}
\usepackage[light, largesmallcaps]{kpfonts}
\usepackage{tikz}
\usepackage{float}
\usepackage{babel}
\begin{document}
\begin{figure}[H]
\centering
\begin{tikzpicture}[scale = 1.2]
\tikzstyle{N}= [circle, minimum size = 2 pt, inner sep = 2 pt, draw = black, fill = yellow]
\draw [line width = 3 pt, color = red, rounded corners] (89.8:5.7) -- (89.8:5.5)
arc [start angle = 89.8, end angle = 83, radius = 5.5] -- (83:5.7)
arc [start angle = 83, end angle = 75, radius = 5.7] -- (70:7)
arc [start angle = 70, end angle = 66.7, radius = 7]
-- (65.5:7.3) ;
\draw [line width = 3 pt, color = green, rounded corners,xscale=-1] (89.8:5.7) -- (89.8:5.5)
arc [start angle = 89.8, end angle = 83, radius = 5.5] -- (83:5.7)
arc [start angle = 83, end angle = 75, radius = 5.7] -- (70:7)
arc [start angle = 70, end angle = 66.7, radius = 7]
-- (65.5:7.3) ;
\end{tikzpicture}
\caption{Une ligne rouge}
rouge et sa symétrique en vert}
\end{figure}
\end{document}
