GitHub recently added the concept of Draft Pull Request: https://github.blog/2019-02-14-introducing-draft-pull-requests/

It seems to be a useful property to expose in this plugin, for example to early abort a build.

The value is not yet available by default in the v3 API though:
https://developer.github.com/v3/pulls/#get-a-single-pull-request

The Draft Pull Request API is currently available for developers to preview. [...] To access the API during the preview period, you must provide a custom media type in the Accept header