We could provide warning-level build feedback e.g. "Notice: Custom plugins are disabled when previewing locally", but I think the better, more descriptive solution would be for Jekyll to warn when a plugin is requested but ignored.

If I forked a project that used the github-pages gem for github deployment but I don't want to github-deploy, how do I deactivate this "feature" of blocking plugins that are forbidden on a platform I'm not using?

I would want bundle exec jekyll serve --trace to print a big fat warning:

"There is files in the _plugins/ folder. Are you trying to use plugins? GitHub only allows a short list of plugins. If you want to override this limitation for local builds, do this little trick: ..."