I understand. Just to make my point clear before we close this, let me explain what I would like to have:
I want to publish notifications (email for example) when a build fails. On that email I want to add information about which pull request the build refers too and its author (I'm only working with pull requests)
AFAIK Jenkins is receiving a webhook containing all that information https://confluence.atlassian.com/bitbucket/event-payloads-740262817.html#EventPayloads-PullRequestEvents
Technically it should be possible to get that information from my pipeline script (without having to hack). Yet, I'm not fully aware of how the pipeline script work internally neither how this plugin is built and I still haven't look into it, but in theory I'm not asking anything weird. But please, enlighten me and feel free to close this.
github-branch-source-plugin is not providing such variables.