siunitx decimals only for numbers below 1

If this is only for use in tables then you can use pgfplotstable and preprocess the numbers. Below I define a new key round int which provides the preprocessing.

\documentclass{article}

\usepackage{siunitx,pgfplotstable}
\pgfplotsset{compat=1.17}

\begin{document}

\makeatletter
\pgfplotsset{table/round int/.style={%
  /pgfplots/table/preproc cell content/.append code={%
    \pgfkeysgetvalue{/pgfplots/table/@cell content}\pgfmathresult
    \ifx\pgfmathresult\pgfutil@empty
    \else
    \pgfmathparse{abs(\pgfmathresult) > 1 ? round(\pgfmathresult) : \pgfmathresult}%
    \pgfkeyslet{/pgfplots/table/@cell content}\pgfmathresult
    \fi}}}
    \makeatother

\pgfplotstabletypeset[multicolumn names,
  columns/0/.style={column name={title},
    string type,
    column type={S[table-alignment=right,
                  round-mode=places,
                  round-precision=1,
                  table-format=-5.1,
                  zero-decimal-to-integer]},
    round int
}
  ]{
11111
11.11
11.85
0.11
0.19
-10.2
-10.8
}

\end{document}

Unfortunately the standard preproc/expr included with pgfplotstable has some settings that are not appropriate for your data. The above code is based on how preproc/expr is implemented in that package.

If you want the numbers right-aligned in the column, just replace the table-format argument to the S column with table-parse-only:

\documentclass{article}

\usepackage{siunitx,pgfplotstable}
\pgfplotsset{compat=1.17}

\begin{document}

\makeatletter
\pgfplotsset{table/round int/.style={%
  /pgfplots/table/preproc cell content/.append code={%
    \pgfkeysgetvalue{/pgfplots/table/@cell content}\pgfmathresult
    \ifx\pgfmathresult\pgfutil@empty
    \else
    \pgfmathparse{abs(\pgfmathresult) > 1 ? round(\pgfmathresult) : \pgfmathresult}%
    \pgfkeyslet{/pgfplots/table/@cell content}\pgfmathresult
    \fi}}}
    \makeatother

\pgfplotstabletypeset[multicolumn names,
  columns/0/.style={column name={title},
    string type,
    column type={S[table-alignment=right,
                  round-mode=places,
                  round-precision=1,
                  zero-decimal-to-integer,
                  table-parse-only]},
    round int
}
  ]{
11111
11.11
11.85
0.11
0.19
-10.2
-10.8
}

\end{document}

If you need to be able to use such a feature both in a table and outside in the text, you can make use of the math features in pgfmath to define the \MyRoundMacro:

\newcommand*{\MyRound}[1]{%
    \pgfmathtruncatemacro{\@IntegerComponent}{abs(#1)}%
    \ifnum\@IntegerComponent=0
        \num[round-mode=places, round-precision=1]{#1}%
    \else
        \num{\@IntegerComponent}%
    \fi
}%

You can either use this directly for each entry in the table or use the collcell package to define a custom column type R and use that:

\newcolumntype{R}{>{\collectcell\MyRound}r<{\endcollectcell}}

where the r is the desired column alignment.

The tables produced by the MWE below are as desired:

Notes:

• Using the R column type requires that you wrap any not numerical content (such as the title lines) within a \multicolumn{1}{c}{}.

Code:

\documentclass{article}
\usepackage{siunitx}
\usepackage{collcell}% Needed only if desire to use the `R` column type defined below
\usepackage{pgfmath}

\begin{document}

\makeatletter
\newcommand*{\MyRound}[1]{%
    \pgfmathtruncatemacro{\@IntegerComponent}{abs(#1)}%
    \ifnum\@IntegerComponent=0
        \num[round-mode=places, round-precision=1]{#1}%
    \else
        \num{\@IntegerComponent}%
    \fi
}%
\makeatother

\newcolumntype{R}{>{\collectcell\MyRound}r<{\endcollectcell}}

In a table the \verb|R| column type (requires non data entries to be wrapped in a \verb|\multicolumn|):

\begin{tabular}{R}
    \multicolumn{1}{c}{\bfseries title}  \\
    11111    \\
    11.11    \\
    0.11     \\
\end{tabular}

Can use the \verb|\MyRound| macro directly in a table and
outside of a table:

\begin{tabular}{r}
    {\bfseries title}  \\
    \MyRound{11111}    \\
    \MyRound{11.11}    \\
    \MyRound{0.11}     \\
\end{tabular}

Outside of a table:

\MyRound{11111}\par
\MyRound{11.11}\par
\MyRound{0.11}\par

\end{document}