i'm actually totally with you on this.
i've been wanting to only build branches (and never build PRs) for a while now. in fact, we've gone so far as to "kill" PR builds manually in our shared library in order to not chew up resources needlessly:
if (env.BRANCH_NAME && env.BRANCH_NAME.contains('PR-')) {
error("IntentionallyFailingPullRequestBuild")
}
our workaround is annoying since our pull requests obviously always show up as broken in bitbucket server. :|
i think
https://issues.jenkins-ci.org/browse/JENKINS-43426 is related, and lots of new use-cases will be supported when this new UI is eventually rolled out (i hope our "ignore PRs completely" use case is supported, but i'm not 100% sure what's in scope).