libheif-js (libheif and libde265 compiled to WebAssembly) 1.23.2 is free software under the GNU Lesser General Public License, version 3 (LGPL-3.0).
Palz Tools serves it unmodified as a separate file and loads it at runtime; it is not part of the site's own code.
Source code: https://github.com/catdad-experiments/libheif-js/tree/1.23.2 (build scripts), https://github.com/strukturag/libheif and https://github.com/strukturag/libde265 (C/C++ sources)
Licence texts (LGPL-3.0 and the GPL-3.0 it refers to): COPYING.txt in this folder.