For a long time, the Isotope jquery library has had a commercial license that made it not really compatible with the GPL.
Isotope v2.2.0 has shipped with revised licensing model that is GPLv3 by default. Purchasing a commercial license allows use outside of the GPL, under the Commercial License terms, without causing conflicts. You can read details at: http://isotope.metafizzy.co/license.html
You can see the whole history here: https://github.com/metafizzy/isotope/issues/800
The tl;dr is this: If you’re using Isotope 2.2 and up in your plugin, it’s permitted in the plugin repository but you need to license YOUR plugin as GPLv3.