Processing core libs are LGPL, which is mostly free to use in commercial projects, but there are still conditions you have to follow. Lots of literature on the license. As long as you keep the libraries intact and replaceable by the end user in the process of obfuscation, you should be OK. yada, yada, yada, IANAL. 
Processing IDE is GPL licensed, hence asking what a plugin for?
Any other third party libraries you bring in may have constraints of their own.
I’ve been coding professionally for almost 20 years - I stand by that comment! 