Look at almost any CTAN package and its license is LPPL — the license of LaTeX itself and the vast majority of packages. It is free software, but with one unusual idea at its core: protecting the guarantee that a file of a given name behaves the same everywhere.
What the LPPL is
The LPPL (The LaTeX Project Public License) is the license under which the LaTeX kernel and standard packages are distributed, written by the LaTeX Project. The current version is 1.3c (released 2008-05-04; 1.3, 1.3a, and 1.3b differ from it only in minor ways). It is a free-software license: the FSF recognizes it as free, it is OSI-approved and Debian (DFSG)-compliant, and its SPDX identifier is LPPL-1.3c. It is, however, incompatible with the GPL — its requirement that modified versions identify themselves as changed (below) is an extra condition the GPL does not permit. It is also the most common license on CTAN.
The core idea — protecting file integrity
Why impose a condition ordinary free licenses lack? Because TeX’s value rests on reproducibility — a given .tex source should produce the same result wherever it is typeset. If someone shipped a modified article.cls under the same name, that guarantee would collapse and you would get “it compiles for me but breaks for you.” The LPPL prevents exactly this.
In the old versions (1.0–1.2) this was a strict “filename clause”: a modified file had to be renamed. Version 1.3 relaxed it — now a modified component must clearly and unambiguously identify itself as modified, both in the source and when run interactively (in logs or banners), and you must document the changes (e.g. ship a changelog). In practice, renaming a fork is still the safest, surest way to comply.
The maintenance model
The LPPL’s other distinctive feature is a mechanism for stating a maintenance status. Every work has a Current Maintainer, and the status is one of three: maintained (someone has taken on upkeep and accepts bug reports), author-maintained (only the original author may maintain it), or unmaintained (no maintainer, or unreachable for six months with no sign of activity). CTAN recommends “maintained” over “author-maintained,” so the work stays useful to the community.
This is what lets the community take over an abandoned package. If a work is unmaintained, the license lays out a clear path: (1) make reasonable efforts to find the current maintainer (e.g. an internet search), (2) if that fails, announce your intent to maintain it in the relevant communities, and (3) if neither the original maintainer nor the copyright holder objects within three months, you may become the new Current Maintainer. It is designed so the ecosystem keeps moving even when an author disappears.
Applying it to your own package
Releasing your own .sty/.cls under the LPPL is straightforward. Put a notice like the one below at the top of each file, license it as “1.3c or any later version,” choose the maintained status, and name the Current Maintainer and the files the work consists of. That is the form CTAN expects (you also pick the license when you upload). Note the LPPL is not public domain — you may use it freely, but the identification and documentation duties above still apply.
% This work may be distributed and/or modified under the
% conditions of the LaTeX Project Public License, either version 1.3
% of this license or (at your option) any later version.
% The latest version of this license is in
% https://www.latex-project.org/lppl.txt
% and version 1.3c or later is part of all distributions of LaTeX
% version 2005/12/01 or later.
%
% This work has the LPPL maintenance status 'maintained'.
% The Current Maintainer of this work is M. Y. Name.
%
% This work consists of the file mypkg.sty.