Back in 2007, Joomla.org decided to stop listing any software that used license keys. They did this because they believed that license keys were incompatible with the GPL license.
Fast forward to 2015 and many (most?) commercial WordPress plugins now use license keys. Some of these license keys just provide access to updates and support – those aren’t the issue here.
However, some license keys lock down the software to a single domain, disabling the plugin even on test sites and localhost installs.
Does this use of license keys violate the GPL?