Linux libertine, the {\l} character, ligatures, and T1 fontenc

Load T1 before OT1, undeclare \l as an OT1 command and declare its default to be T1:

\documentclass{article}
\usepackage[T1,OT1]{fontenc}
\usepackage{libertine}

\UndeclareTextCommand{\l}{OT1}
\DeclareTextSymbolDefault{\l}{T1}

\title{This is Quantifiably finally ligatured text}
\begin{document}

\maketitle
I need the character \l{} too though.
\end{document}

Of course you lose some kerning pairs and hyphenation in words containing \l, but I don't think it's a big problem.

Here's what I get after copying the glyph from the PDF viewer window and pasting it in Unicode Checker

Update

As of the version of libertine released on 2017/03/20, the \l and \L commands do the right thing also with the OT1 encoding.

\documentclass{article}
\usepackage{libertine}

\title{This is Quantifiably finally ligatured text}
\begin{document}
\maketitle
I need the characters \l{} and \L{} too though.
\end{document}

Another method, by redeclaring \l to be in T1 font encoding:

\documentclass{article}
\usepackage[OT1]{fontenc}
\usepackage{libertine}

\let\oldl\l
\renewcommand{\l}{\begingroup\fontencoding{T1}\selectfont\oldl\endgroup}

\title{This is Quantifiably finally ligatured text}
\begin{document}

\maketitle
I need the character \l{} too though.
\end{document} 

Output: