subcaptionphantom side_by_side comparison (for rev3) https://tex.stackexchange.com/a/468902/143086

subcaptionphantom_macros.tex

% =============== defining the \subcaptionphantom ===================================
\makeatletter
% \begin{macro}{\subcaptionphantom}
% \cs{subcaptionphantom}\oarg{list-entry}\marg{caption} is a hidden
% non-printed subcaption. Designed for the case if "(a)", "(b)" are
% already embedded in the figure itself. 
% Roughtly equivalent to \cs{phantomsubcaption} from the \Lpack{subcaption} package.
%
%    \begin{macrocode}
\newcommand{\subcaptionphantom}{%
  \bgroup    
    \let\label=\memsub@label
    \ifdonemaincaption\else
      \advance\csname c@\@captype\endcsname\@ne
    \fi
    \refstepcounter{sub\@captype}\@contkeep
    \@ifnextchar [%
      {\@memsubcapphantom{sub\@captype}}%
      {\@memsubcapphantom{sub\@captype}[\@empty]}}
%    \end{macrocode}
% \end{macro}

% \begin{macro}{\@memsubcapphantom}
% Quick-and-dirty analog of \Lpack{memoir} \cs{memsubcap}, adapted
% for use in \cs{subcaptionphantom}.
%    \begin{macrocode}
\long\def\@memsubcapphantom#1[#2]#3{%
  \@tempdima=\hsize
  \vskip\subfloatcapskip
  \ifx \@empty #2
    \@memsubcaptionphantom{#1}{#3}{#3}%
  \else
    \@memsubcaptionphantom{#1}{#2}{#3}%
  \fi
  \vskip\subfloatcapskip
  \egroup}
%    \end{macrocode}
% \end{macro}
%
%
% \begin{macro}{\@memsubcaptionphantom}
% Quick-and-dirty analog of \Lpack{memoir} \cs{memsubcaption}, adapted
% for use in \cs{memsubcapphantom}.
%    \begin{macrocode}
\newcommand{\@memsubcaptionphantom}[3]{%
	
  \ifx \relax#2\relax \else
    \bgroup
      \let\label\@gobble
      \let\protect\string
      \def\@memsubcaplabel{\@nameuse{@@the#1}}%
      \xdef\@memsubfigcaptionlist{%
        \@memsubfigcaptionlist,%
  {\protect\numberline{\@memsubcaplabel}\noexpand{\ignorespaces #2}}}%
    \egroup
  \fi
  \@makesubfloatcaptionphantom{\@nameuse{@the#1}}{#3}%
  }

%    \end{macrocode}
% \end{macro}

% \begin{macro}{\@makesubfloatcaptionphantom}
% Quick-and-dirty analog of \Lpack{memoir} \cs{makesubfloatcaption}, adapted
% for use in \cs{memsubcaptionphantom}.
%    \begin{macrocode}
\newcommand{\@makesubfloatcaptionphantom}[2]{%
  \setbox\@tempboxa\hbox{%
    \@subcapsize
    {\phantom{\@subcaplabelfont#1}}{\ignorespaces #2}\unskip}%
  \@tempdimb=-\subfloatcapmargin
  \multiply\@tempdimb\tw@
  \advance\@tempdimb\@tempdima
  \hb@xt@\@tempdima{%
    \hss
    \ifdim \wd\@tempboxa >\@tempdimb
      \phantom{\memsubfig@caption{#1}}{#2}%
    \else
      \if@shortsubcap
        \phantom{\memsubfig@caption{#1}}{#2}%
      \else
        \box\@tempboxa
      \fi
    \fi
    \hss}}
%    \end{macrocode}
% \end{macro}
\makeatother
% ===================================================================================