Extended \Mapsto with text above

The mathtools package provides nearly everything you need to construct this, and stmaryrd has the extra pieces of symbols. Putting these together gives

\documentclass{article}

\usepackage{centernot}
\usepackage{mathtools}
\usepackage{stmaryrd}

\makeatletter
\newcommand{\xMapsto}[2][]{\ext@arrow 0599{\Mapstofill@}{#1}{#2}}
\def\Mapstofill@{\arrowfill@{\Mapstochar\Relbar}\Relbar\Rightarrow}
\makeatother

\begin{document}
\begin{gather*}
   \mapsto \qquad \xmapsto[A]{sdfkjhsdf} \\
 \Mapsto \qquad \xMapsto[A]{sdfkjhsdf}
\end{gather*}
\end{document}

The parameters 0599 are for the placement of the labels and how much the arrow extends over them. I have use the same numbers as mathtools does for \xmapsto. \Mapstochar is the short bar at the beginning, from the stmaryrd package, \Relbar is the double line which fits with final \Rightarrow. The internal macros names include the @ character, so have to be included in a \makeatletter / \makeatother pair.

It might have been nice to use the extpfeil package for this, but unfortunately it loads stmaryrd with clashing options.

The fp package is used to calculate the appropriate width of the arrow. The kerns will be important if you change fonts. It's set up to provide an arrow length of a certain minimum width, regardless of the over/underset. THen, as the over/underset grows, the arrow width will grow with it.

\documentclass{article} 
\usepackage{stackengine}
\usepackage{scalerel}
\usepackage{fp}
\def\clipeq{\kern -.65pt\mathrm{=}\kern -1pt}
\def\Lla{\rule[-.1ex]{.3pt}{1.3ex}}
\def\Lra{\kern -3.5pt\Longrightarrow}
\newcount\argwidth
\newcount\clipeqwidth
\savestack\tempstack{\stackon{$\clipeq$}{}}%
\clipeqwidth=\wd\tempstackcontent\relax
\newcommand\xMapsto[2]{%
  \savestack\tempstack{\stackon{$\scriptstyle#1$}{$\scriptstyle#2$}}%
  \argwidth=\wd\tempstackcontent\relax%
  \FPdiv\scalefactor{\the\argwidth}{\the\clipeqwidth}%
  \FPsub\scalefactor{\scalefactor}{1.5}% <---CAN PLAY WITH THIS VALUE
  \FPmax\scalefactor{\scalefactor}{.05}%
  \mathrel{%
  \stackunder[2pt]{\stackon[3pt]{$\Lla\hstretch{\scalefactor}{\clipeq}\Lra$}%
     {$\scriptstyle#1~$}}{$\scriptstyle#2~$}%
  }%
}
\parskip 1ex
\begin{document} 
$X \xMapsto{ABC}{defghi} Y$

$\xMapsto{}{}$

$\xMapsto{a}{}$

$\xMapsto{ab}{}$

$\xMapsto{abc}{}$

$\xMapsto{abcd}{}$

$\xMapsto{abcde}{}$
\end{document}