The plugin directory’s licensing guidelines have been updated. The guidelines will now allow code that is licensed under (or compatible with) version 3 of the GPL.
The guidelines still encourage use of “GPLv2 or later,” the same license as WordPress. However, we understand that many open source libraries use other licenses that are nonetheless compatible, such as GPLv2 only, GPLv3, and Apache 2.0.
Now may be a good time for plugin authors to review their plugins to ensure a license is specified. You can add License and License URI headers to readme.txt and the plugin’s headers. (You may also wish to include a copying permission statement.) For example:
License: GPLv2 or later
License URI: http://www.gnu.org/licenses/gpl-2.0.html
You can see this used in the sample readme.txt.
This change brings the guidelines in line with the themes directory, which has for some time accepted GPLv3-compatible code. (Probably a good time to note that Creative Commons licenses are still incompatible with the GPL, and the theme and plugin directories.)
WordPress has always stated that its license was the GNU GPL, and has bundled version 2 of the license, even though no GPL version was specified. The text of version 2 (as well as 1 and 3) says that if no version is specified, the software can be redistributed under the terms of any version of the GPL published by the FSF.
However, WordPress contains libraries which are licensed under the GPL “version 2 or any later version,” which obviously excludes version 1 of the GPL. Here is the reality: the GPL version 1 is effectively irrelevant. It hasn’t been a commonly used license since before Matt Mullenweg was in third grade! Clarifying WordPress as being licensed under the GPL “version 2 or later” resolves these niggling library licensing concerns or ambiguities, and clarifies where WordPress stands.
It was the intention of the WordPress founders and developers to be GPL version 2 or later from the beginning, and we have now made that license properly explicit.