The absent-minded linguist

No, as you've noted, it was abandoned in 2010, and is completely unsupported and thoroughly outdated at this point in time. You should definitely look somewhere else, and where exactly depends on your specific needs.

I think I have almost the solution with the help of two questions: Create a contingency table using pgfplotstable and Accessing individual table elements with pgfplots? The idea is, to transpose the matrix, create a new matrix, loop over all columns and copy the columns, if the Exmpl - value is 1.

\documentclass{article}
\usepackage{array}
\usepackage{pgfplots}
\pgfplotsset{compat=1.5.1}
\usepackage{pgfplotstable}
\usepackage{ifthen}

\newcommand{\pgfplotstablefilterrows}[3]
{
  \pgfplotstablegetrowsof{#1}
  \pgfmathsetmacro{\NumOfRows}{\pgfplotsretval}
  \pgfmathsetmacro{\MaxRow}{\NumOfRows-1}
  \pgfplotstablegetcolsof{#1}
  \pgfmathsetmacro{\NumOfCols}{\pgfplotsretval}

  \pgfplotstabletranspose{\TransposedData}{#1}
  \pgfplotstableset{create on use/TransposedHead/.style={copy column from table={\TransposedData}{[index]0}}}
  \pgfplotstablenew[columns={TransposedHead}]{\NumOfCols}{\TransposedFilteredData}
  \pgfplotsforeachungrouped \pgfplotstablerowindex in {0,1,...,\MaxRow}{ % Row loop
    #3
  }
  \pgfplotstabletranspose[colnames from=TransposedHead,input colnames to=]{#2}{\TransposedFilteredData}
  \pgfplotstableclear{\TransposedData}
  \pgfplotstableclear{\TransposedFilteredData}
}

\begin{document}
  \pgfplotstableread{
    Exmpl  a   v
    1      0   0
    1      1   1
    1      2   1
    1      3   4
    2      0   -0
    2      1   -1
    2      2   -1
    2      3   -4
  } \data

  \pgfplotstablefilterrows{\data}{\FilteredData}
  {
    \pgfplotstablegetelem{\pgfplotstablerowindex}{[index]0}\of\data
    \ifthenelse{\equal{\pgfplotsretval}{1}}
    {
      \pgfplotstablecreatecol[copy column from table={\TransposedData}{\pgfplotstablerowindex}]{\pgfplotstablerowindex}{\TransposedFilteredData}
    }
    {}
  }

  \begin{tikzpicture}
    \begin{axis} % [legend pos=outer north east]
      \addplot table [x=a, y=v] {\data};
      \addplot table [x=a, y={create col/linear regression={y=v}}] {\FilteredData}; % compute a linear regression from the input table
    \end{axis}
  \end{tikzpicture}
\end{document} 

As you might see from the code, I'm not really experienced in latex programming. Anyway, it works. Best, Juhui