I've thought some about this sort of thing - I'm considering maybe adding a finally post condition (which would also be helpful for JENKINS-41519, I think) that would always be run, regardless of build status, stage execution, etc, and would always be run after all other post conditions. But I'm not sure that directly addresses your use case - maybe a skipped post condition?

In general, I really need to rewrite post entirely behind the scenes - there are a bunch of limitations its current implementation creates. Sigh. =)

A finally post would suffice if I could get the status (e.g. failure, success, skipped). However, the resulting conditions would contradict the descriptive nature, I think.

As I understand (in addition to your comment in JENKINS-47577) that you propose it like this?

stage ('foo') {
  when {
    ...
  }
  steps {
    ...
  }
  post {
    ...
    # run as usual when the stage is not skipped because of the when condition
  }
  skipped {
    ...
    # run when the stage is skipped because of the when condition
  }
  finally {
    ...
    # run in all cases after post/skipped or only after post when not skipped?
  }
}

Any news on this? I would find this useful too.

I'd also like to point out that the documentation for the post block's cleanup condition is unclear and could be taken to mean it will run when a stage is skipped even though it doesn't.  If you interpret 'Skipped' as a possible stage status  (i.e. stageResult=NOT_BUILT I believe)

cleanup
Run the steps in this post condition after every other post condition has been evaluated, regardless of the Pipeline or stage’s status.

https://www.jenkins.io/doc/book/pipeline/syntax/#post

As much as I hate bumps and their ilk, I'm also wondering if any effort has been made in this direction.  Much like the OP, I want to update GitHub's commit status checks with a skipped status for skipped stages rather than having them sit pending indefinitely.