I don't believe there is webhook support for any package repository plugins. The language is generic on the UI independent of material type, which is why it mentions webhooks.
I'm not really sure how such a webhook would be technically possible for package repositories, given the variation of types and lack of standardised API to do so - I guess it'd have to be
Nexus specific, and GoCD itself is agnostic to package type and repository manager, so there'd have to be a way for a plugin to offer a webhook API somehow to implement Nexus/Artifactory/whatever specifics.
-Chad