A way to manage untrusted branches was introduced in scm-api 1.1 (and used by workflow-multibranch 1.15).
Implementing this API will allow to enable pull requests auto-discovering for public repositories (currently limited to private).
See this implementation as reference.
Allow SCMSource and SCMNavigator subtypes to share common traits
I believe JENKINS-43507 contains this functionality and is due to be released in a few weeks.