\documentclass[a4paper]{memoir}
\usepackage{xparse,amsmath}
\ExplSyntaxOn
\clist_new:N \l_daleif_tsub_clist
\NewDocumentCommand\tsub{ m }
 {
  \sb {
    \clist_set:Nn \l_daleif_tsub_clist {#1}
    \clist_pop:NN \l_daleif_tsub_clist \l_tmpa_tl
    \textup{\l_tmpa_tl}
    \clist_if_empty:NF \l_daleif_tsub_clist
     {
      , \clist_use:Nn \l_daleif_tsub_clist{,}
     }
  }
 }
\ExplSyntaxOff

\begin{document}
\[
X\tsub{max} \qquad X\tsub{max,d,t,t}
\]
\end{document}

\RequirePackage{scrlayer-scrpage} % Required to customise headers and footers
\PreventPackageFromLoading[\ClassError{\classname}{package
`fancyhdr'  incompatible with this class}{The pagesyles
are defined using package scrlayer-scrpage, ^^Jplease consult the
KOMA-script documentation for details.}]{fancyhdr}

\documentclass{scrartcl}
\usepackage{kantlipsum}
\begin{document}
\kant*[7]

\begin{figure}[!htbp]
  \leavevmode\smash{\rule{1pt}{26pt}}\kant*[7]
\end{figure}

\leavevmode\smash{\rule{1pt}{26pt}}\kant*[7]
\end{document}