You can modify the GPL with an addition term that requires the use of green energy, but the resulting license is incompatible with the GPL with the result that code under the new license cannot be mixed with or linked with GPLed code. See section 7 ("additional terms") of the GPL:
The reason why it fell mostly in disuse nowadays is because jshint appeared, which allowed a lot more customization of the available rules being applied.
Later eslint popped up with it's plugin engine allowing anyone to have their own custom set of rules.
As far as I know, no one cared about jshint's license because there was no code being shipped, and eventually code would be minified anyway leaving no traces of jshint.
Now... We can talk about the morality of ignoring a license, but ultimately jshint's license was simply ignored by almost everyone
https://www.gnu.org/licenses/gpl-3.0.en.html
(the GPL version 2 also has this property.)
Also, its unusual license is probably why jslint is not used much.