it is possible that GitHub Branch Source has a regression, but https://github.com/jenkinsci/github-branch-source-plugin/blob/master/src/test/java/org/jenkinsci/plugins/github_branch_source/GitHubSCMFileSystemTest.java#L193 is passing (yes based on an evil replayed capture of real github), so unless https://github.com/jenkinsci/github-branch-source-plugin/blob/master/src/main/java/org/jenkinsci/plugins/github_branch_source/GitHubSCMProbe.java#L135-L151 has a bug, we should be detecting the Jenkinsfile at the path and creating the branch, and then loading the file from the lightweight checkout (which the test "claims" is working)

So it looks like GitHub is working with child paths... of course I don't trust the wiremock tests and would love to rip them out... but GitHub has that rate limit that makes testing live a nightmare!

Well, unfortunately it does not, as confirmed by Vivek and myself.

This is still an issue and breaking projects. 

I can confirm that the original issue is still there. I don't know if it is in Bitbucket branch source plugin or elsewhere. I have tested 2.2.10 (which is the latest at the time of writing) with Bitbucket Server and Jenkinsfile placed in build/test.jenkinsfile in my repository. With 'master' branch it is flawless. However, when I put it to branch 'feature/test', in the console output I have:

Lightweight checkout support not available, falling back to full checkout.
Checking out git ssh://localhost:7999/bitbucket/~atikhonova/test.git into /var/lib/jenkins/workspace/test@script to read build/test.jenkinsfile

The workaround is to put 'refs/heads/feature/test' in Branch Specifier field of Git SCM. Not 'feature/test', not '*/feature/test'. You have to specify your git ref at full as this is the only supported case. See the breaking code in git-plugin (src/main/java//jenkins/plugins/git/GitSCMFileSystem.java):

@Extension(ordinal = Short.MIN_VALUE)
public static class BuilderImpl extends SCMFileSystem.Builder {

  @Override
  public boolean supports(SCM source) {
    return source instanceof GitSCM
      && ((GitSCM) source).getUserRemoteConfigs().size() == 1
      && ((GitSCM) source).getBranches().size() == 1
      && ((GitSCM) source).getBranches().get(0).getName().matches(
      "^((\\Q" + Constants.R_HEADS + "\\E.*)|([^/]+)|(\\*/[^/*]+(/[^/*]+)*))$"        // HERE !!
    );
  // we only support where the branch spec is obvious

I think this simple implementation is good and the only issue here is the match failing silently. Probably, this method should put a warning to the Console explaining why lightweight checkout failed.