How to type the letter Ł

The traditional markup (going back to plain TeX) is \L in current releases you can simply use Ł

\documentclass{article}

\begin{document}

Ł 

\end{document}

Prior to the 2018 LaTeX release you would need to explicitly load inputenc as shown below to be able to enter it as is. fontenc makes it one character but is not necessary to input it (it'll get displayed by a character "L" and an overlapping dash without fontenc). As noted in the comments \L is the underlying macro that is used for the character, so you could also use \L in every occurrence of Ł.

\documentclass[]{article}

\usepackage[utf8]{inputenc}
%\usepackage[T1]{fontenc}

\begin{document}
Ł
\end{document}

Without fontenc:

With fontenc:

If you're using XeLaTeX or LuaLaTeX you don't need the inputenc packages as they are unicode enabled by default.

For example, you can type:

Łojasiewicz
\bye

and use the command csplain or pdfcsplain to process the document.

Quick answer:

\L{} or \l{}

Reference: https://latex.org/forum/viewtopic.php?t=2239