The wording "should" is a dead giveaway. The FSF would generally like you (and everybody else) to comply with their ideology but they have no real means of forcing you, in general. That's not necessarily a bad thing because although it's a bit quixotic, the FSF ideology isn't bad.
In general, you are legally bound to comply with all terms that anyone (not just the FSF) puts in a license (not just GPL) if you do something, anything, that makes you subject to the license.
If you do not do anything that makes you subject to the license, people may still have opinions and wishes, but they are just that. Much like vegans tell you every day how eating animals is bad, and that you "should" not do it, but they have no means of enforcing their ideology.
In the case of GPL, in order not to be bound by its terms (all of them!), what you need to avoid is, in very simplified and somewhat inaccurate words, distributing a program that includes, links with, or derives from code which is under that license.
In some people's opinion, plugins are by their very nature derivative work since they cannot work without the host application. While there is a grain of truth in that argument, I nevertheless beg to differ on that opinion for the general case.
A plugin needs a host application to run, that's true. However, this needs not necessarily be one particular application, or even an application that is under some particular license (including GPL). Also, a plugin hopefully adds something to the program rather than just exposing functionality that is already present anyway.
Examples of plugins that will very well work with different applications can be found in the audio/video and graphics world. The same, identical plugins will work interchangeably with different programs. Plugins written in embedded scripting languages would be another example, they are not necessarily bound to one particular application.
Another example where the logic doesn't hold would be dual-licensed software. For example, you can have MySQL under two different licenses where one is GPL, and the other is not. MySQL supports plugins. So, whether or not your plugin falls under GPL depends on whether some future client buys a commercial license from Oracle? And your obligations are different for everybody else? Or only until the first person buys a commercial license? Well, no, that's hardly how it is...
So, in my opinion, the idea that a plugin is necessarily a derivative work doesn't hold. It may be, can be, but doesn't need to be.
Note, however, that a plugin needs to follow some kind of specification (some particular file format, magic numbers, exported symbols, etc) and possibly use an API of sorts to connect itself to the application. Which may require e.g. including some headers (that contain inline functions) or linking to some libraries. Or, whatever.
These may or may not be licensed under the GPL like the application, or they may be under the GPL, but with an explicit exception. You will need to verify to be sure.
Also, it is important to note that the FSF doesn't own the rights on the software (nor do I, or any other random person here), so whatever they think is widely irrelevant. What the actual author(s) has/have in mind is what's relevant.
Therefore, when in doubt about the exact intent, it is a good idea not to ask FSF or a Q&A site, but to contact the authors. A lawyer cannot really help you either (the lawyer can express an opinion, but although this opinion is very expensive, it is also entirely worthless).
Ask the authors what their intent is. Quite likely, you will get an answer like "Wait, what? No, of course that's not intended, please go ahead, write your plugin!".
That's because plugins, whether open-source or not, greatly enhance your software's visibility, exposure, and overall value. Few authors will turn that down for ideology. But if they do turn you down, you know. That's more than anyone else (me, someone else on this site, or your lawyer) can tell you.
Get an authorative reply from the authors / copyright holders, it's the only thing that matters. Whatever is written in some license document or one someone's website is entirely irrelevant, since they simply do not have a say. The only ones who do have a say are the authors.