¿Cómo crear un entorno entre corchetes como este?

Basta ya de investigar por todo internet ya que has llegado al sitio adecuado, tenemos la respuesta que buscas sin liarte.

Solución:

Solo por diversión: un tcolorbox respuesta basada.

documentclassarticle
usepackage[most]tcolorbox
usepackagelipsum
newtcolorboxlbulbox[1][]enhanced,
colback=white,
boxrule=0mm,top=0mm,bottom=0mm,left=2cm,right=4mm,sharp corners,
overlay=%
draw[ultra thick] ([xshift=2mm]frame.north west)-

begindocument
lipsum[1]
beginlbulbox
textbfNew- and old-style classes.par
Old-style classes have only human students. In new-style classes, ducks, koalas
and marmots are welcome. Hibernation needs of marmot students are respected. 
endlbulbox
lipsum[2]
enddocument

La siguiente es una implementación rápida de lo que desea sin ningún paquete. La sintaxis es rápida y sucia y una mezcla de sintaxis LaTeX y TeX. Cada aspecto del entorno debe ser ajustable si cambia las longitudes y macros utilizadas (cada una debe comentarse para dejar en claro para qué se usa).

documentclass[]article

usepackage[]graphicx

makeatletter
newcommand[email protected]





[1]textbf#1 % format the heading
% store the icon
newsavebox[email protected]
setbox[email protected]hboxincludegraphics[width=1cm]example-image-duck
% the vertical displacement of the icon from the centre of the contents
newlength[email protected]
[email protected]
% store the contents of the environment
newsavebox[email protected]
% the width it should take as a macro so that the width is evaluated when used,
% this should provide more flexibility
newcommand[email protected].95linewidth
% the actual contents width (gets calculated)
newlength[email protected]
% the vertical amount of space the brackets are taller than their contents
newlength[email protected]
[email protected]
% the width of the horizontal rules of the brackets
newlength[email protected]
[email protected]
% the thickness of the bracket rules
newlength[email protected]
[email protected]
% the amount of white space between the left bracket and the logo
newlength[email protected]
[email protected]
% the amount of white space between the logo and the contents
newlength[email protected]
[email protected]
% the amount of white space between the right bracket and the contents
newlength[email protected]
[email protected]
% the vertical white space added above the whole environment
newlength[email protected]
[email protected]
% the vertical white space added below the whole environment
newlength[email protected]
[email protected]
% the definition of the environment
newenvironmentBracket[1]
  %
    [email protected][email protected]
    advance[email protected][email protected]
    advance[email protected][email protected]
    advance[email protected][email protected]
    advance[email protected]wd[email protected]
    advance[email protected][email protected]
    setbox[email protected]vboxbgroup
      hsize[email protected]
      linewidthhsize
      noindent
      [email protected]#1par
      @afterindentfalse
      @afterheading
  
  %
    egroup
    par
    vskip[email protected]
    noindent
    null
    hskipdimexpr(linewidth-[email protected])/2relax
    rlap
      %
        vrule height dimexprht[email protected]+[email protected]
               depth dimexprdp[email protected]+[email protected]
               width [email protected]
        rlap
          %
            rlap
              %
                raiseboxdimexprht[email protected]+[email protected]relax
                  %
                    vrule height 0pt
                           depth [email protected]
                           width [email protected]
                  %
              %
            raisebox-dimexprdp[email protected]+[email protected]relax
              %
                vrule height [email protected]
                       depth 0pt
                       width [email protected]
              %
          %
        hskip[email protected]
        raisebox
          %
            dimexpr
              .5ht[email protected]dp[email protected]
              -.5ht[email protected]+.5dp[email protected]
              +[email protected]
            relax
          
          usebox[email protected]%
        hskip[email protected]
        usebox[email protected]
        hskip[email protected]
        vrule height dimexprht[email protected]+[email protected]
               depth dimexprdp[email protected]+[email protected]
               width [email protected]
        rlap
          %
            hskip-[email protected]
            llap
              %
                raiseboxdimexprht[email protected]+[email protected]relax
                  %
                    vrule height 0pt
                           depth [email protected]
                           width [email protected]
                  %
              %
            llap
              %
                raisebox-dimexprdp[email protected]+[email protected]relax
                  %
                    vrule height [email protected]
                           depth 0pt
                           width [email protected]
                  %
              %
          %
      %
    par
    vskip[email protected]
  
makeatother

usepackageduckuments% only for blindduck

begindocument
blindduck[1]
beginBracketNew- and old-style classes
  blindduck[2]
endBracket
noindent
blindduck[3]
enddocument

EDITAR: Permite un ancho total mayor que el circundante linewidth sin cajas demasiado llenas.

Debo mencionar que el resultado no se puede romper en la página. Si esto es necesario, tcolorbox se puede romper la página si se configura correctamente, así que eche un vistazo a la respuesta de @ marmot (esto no afirma que la forma en que @marmot configuró tcolorbox es la página que se puede romper, no lo comprobé, por lo que podría ser rompible, pero también podría no serlo).

Te mostramos las reseñas y valoraciones de los lectores

Al final de la post puedes encontrar las interpretaciones de otros desarrolladores, tú incluso puedes insertar el tuyo si te apetece.

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