Are these plugins that are part of a package or ones you're installing from elsewhere?

Gajim has a plugin manager, which show you a list of available plugin as soon as you open it (they are developed by the same developers as the main application). You can install them by checking their related checkboxes. Theses plugins are not available as separate arch package. They are only available through the gajim plugin manager (what is fine, though). Some of them require external python lib, which are not dependencies of the main package.

The problem is the following. I want to install the plugin A. A need the python pygments lib to works as expected. I have to figure out by myself that on archlinux this lib is available through the python2-pygments package. I install it. Some time later, as I want to cleanup a bit my linux box, I look at the libs I installed, which seems to be needed by nobody. I may uninstall them, as I have no way to remember why this specific lib is here. Hence the proposal to mark them as optional dependency in order to have gajim listed as optional target and help us remember why we install it.

This isn't really feasible unless the author has a specific set of libs that they use for plugins. You don't know if you have them all, and they could change at any time independent of the package.

Thus I think this ticket can be closed as Won't fix.