Barbara,
I don’t think that GPL requires a creator to keep their plugin “working” even after a license has expired. All GPL really requires is that the code of the plugin be available to the user of the plugin.
In WordPress, this requirement is pretty much always met for plugins, which are shared as source code and installed on your WP installation. Even if the plugin expires, you are welcome to go to your filesystem and review the code, or even rewrite it.
That said, there are ways for plugin authors to compile or obscure portions of their code so that the user really cannot review them. In those cases, I think GPL would require that the uncompiled version of the code be somewhere available to the user of the code. I have seen plugins that do not really take that to heart.
Also, I am not a lawyer nor an expert in GPL. Also, not all plugins are licensed via GPL (mine, for example, are usually distributed under the MIT license). So take all I say here with a large grain of salt!
...Eric
Eric Celeste /
e...@clst.org /
651-323-2009