Following Werner's comment, which links to:
I end up with the following, which (when one would like a left-curly brace) however results in the left brace being oriented falsely (it should be flipped horizontally).
A correct way for the right curly brace can be found in the mentioned User Cmhuges' answer.
breakable, % allow page breaks
frame hidden, % hide the default frame
% right line
(frame.north west)--(frame.south west);
% paragraph skips obeyed within tcolorbox
\item Second line
\item Third line, which is quite long and seemingly tedious in the extreme
\item Fourth line, which isn't as long as the third