mjhjmtu

I'm hoping to write a custom environment for a double-bracketed "key point" section (see image below). The left and right brackets should be as tall as the text and there should be a little icon with title and text, exactly like in the following example:

Not sure where to start.

Just for fun: a tcolorbox based answer.

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

The following is a quick implementation of what you want without any packages. The syntax is quick and dirty and a mixture of LaTeX and TeX syntax. Every aspect of the environment should be adjustable if you change the used lengths and macros (every one should be commented to make clear what it's used for).

documentclassarticle

usepackagegraphicx

makeatletter
newcommandBracket@head[1]textbf#1 % format the heading
% store the icon
newsaveboxBracket@icon
setboxBracket@iconhboxincludegraphics[width=1cm]example-image-duck
% the vertical displacement of the icon from the centre of the contents
newlengthBracket@iconOffset
Bracket@iconOffset0pt
% store the contents of the environment
newsaveboxBracket@box
% the width it should take as a macro so that the width is evaluated when used,
% this should provide more flexibility
newcommandBracket@totwidth.95linewidth
% the actual contents width (gets calculated)
newlengthBracket@width
% the vertical amount of space the brackets are taller than their contents
newlengthBracket@vpad
Bracket@vpad5mm
% the width of the horizontal rules of the brackets
newlengthBracket@length
Bracket@length5mm
% the thickness of the bracket rules
newlengthBracket@linewidth
Bracket@linewidth2pt
% the amount of white space between the left bracket and the logo
newlengthBracket@hpadLeft
Bracket@hpadLeft2.5mm
% the amount of white space between the logo and the contents
newlengthBracket@hpadIcon
Bracket@hpadIcon2.5mm
% the amount of white space between the right bracket and the contents
newlengthBracket@hpadRight
Bracket@hpadRight5mm
% the vertical white space added above the whole environment
newlengthBracket@aboveSkip
Bracket@aboveSkip1ex
% the vertical white space added below the whole environment
newlengthBracket@belowSkip
Bracket@belowSkip1ex
% the definition of the environment
newenvironmentBracket[1]
 %
 Bracket@widthBracket@totwidth
 advanceBracket@width-Bracket@hpadRight
 advanceBracket@width-Bracket@hpadLeft
 advanceBracket@width-Bracket@hpadIcon
 advanceBracket@width-wdBracket@icon
 advanceBracket@width-2Bracket@linewidth
 setboxBracket@boxvboxbgroup
 hsizeBracket@width
 linewidthhsize
 noindent
 Bracket@head#1par
 @afterindentfalse
 @afterheading
 
 %
 egroup
 par
 vskipBracket@aboveSkip
 noindent
 null
 hskipdimexpr(linewidth-Bracket@totwidth)/2relax
 rlap
 %
 vrule height dimexprhtBracket@box+Bracket@vpad
 depth dimexprdpBracket@box+Bracket@vpad
 width Bracket@linewidth
 rlap
 %
 rlap
 %
 raiseboxdimexprhtBracket@box+Bracket@vpadrelax
 %
 vrule height 0pt
 depth Bracket@linewidth
 width Bracket@length
 %
 %
 raisebox-dimexprdpBracket@box+Bracket@vpadrelax
 %
 vrule height Bracket@linewidth
 depth 0pt
 width Bracket@length
 %
 %
 hskipBracket@hpadLeft
 raisebox
 %
 dimexpr
 .5htBracket@box-.5dpBracket@box
 -.5htBracket@icon+.5dpBracket@icon
 +Bracket@iconOffset
 relax
 
 useboxBracket@icon%
 hskipBracket@hpadIcon
 useboxBracket@box
 hskipBracket@hpadRight
 vrule height dimexprhtBracket@box+Bracket@vpad
 depth dimexprdpBracket@box+Bracket@vpad
 width Bracket@linewidth
 rlap
 %
 hskip-Bracket@linewidth
 llap
 %
 raiseboxdimexprhtBracket@box+Bracket@vpadrelax
 %
 vrule height 0pt
 depth Bracket@linewidth
 width Bracket@length
 %
 %
 llap
 %
 raisebox-dimexprdpBracket@box+Bracket@vpadrelax
 %
 vrule height Bracket@linewidth
 depth 0pt
 width Bracket@length
 %
 %
 %
 %
 par
 vskipBracket@belowSkip
 
makeatother

usepackageduckuments% only for blindduck

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

EDIT: Allows a total width greater than the surrounding linewidth without overfull boxes.

I should mention that the result is not page breakable. If this is required, tcolorbox can be page broken if set up correctly, so take a look at @marmot's answer (this doesn't claim that the way @marmot set up tcolorbox is page breakable, I didn't check this, so it might be breakable, but might as well be not).