\count255 \space 3
\showthe\count255

\count255 \space\space 4
\showthe\count255
\count255 \space\space\space 5
\showthe\count255

\count255 = 6
\showthe\count255

\count255 = \space 7
\showthe\count255

:50691945 No strong opinions on that. I might want to include lua in the name if using the package required writing lua, but not if the use of lua is merely an implementation issue, with the package interface still being tex commands.

\documentclass{article}
\usepackage{fontspec,lipsum}
\setmainfont{Corbel}
\usepackage{lipsum}
\linespread{0.8}
\begin{document}
Testing\textsubscript{123}\\%
%Testing$_{123}$\\%
\lipsum
\end{document}

\documentclass{article}
\usepackage{fontspec,lipsum}
\setmainfont{Corbel}
\usepackage{lipsum}
\linespread{0.8}

\begin{document}
\showoutput\showboxdepth3
\showthe\baselineskip
Testing\textsubscript{123}\\%
%Testing$_{123}$\\%
\lipsum
\end{document}

...\hbox(6.5625+2.91159)x345.0, glue set 289.5127fil []
...\penalty 150
...\glue(\lineskip) 1.0
...\hbox(7.07031+2.39256)x345.0, glue set 0.62704 []

...\hbox(6.5625+2.39256)x345.0, glue set 287.234fil []
...\penalty 150
...\glue(\baselineskip) 0.13716
...\hbox(7.07031+2.39256)x345.0, glue set 0.62704 []