Your guess is correct, gems
key is now plugins
.
In jekyll/lib/jekyll/configuration.rb
you can find:
def backwards_compatibilize
...
renamed_key "gems", "plugins", config
It is part of this Pull Request: plugins config key as replacement for gems (#5130)
And it was the result of this issue:
What do you think about renaming the gems config directive to plugins,
with full backwards compatibility?
The idea being, if you're not from the Ruby world, "gems" isn't a very
intuitive term (even though that's what they are). Further, users
shouldn't have to care about (or see) the behind-the-scenes plumbing,
in terms of how things are distributed.
Instead, let's call the key "plugins", a term which has a broader
understanding outside the Ruby ecosystem, and one that should be
familiar to any user coming from a more traditional CMS like WordPress
or Drupal.
In terms of practicality, it'd just be a matter of combining the gems
and plugins arrays, if they exist when we load the config, and
updating the docs in the next minor release.