I am tempted to say that the return value should just be null (unused). If you actually need to track the result of various branches, it is clearer to do so directly. For example:

def x, y
parallel one: {x = 1}, two: {y = 2}
echo "x+y = ${x+y}"

This also makes it obvious how to track exceptions in branches:

def result
parallel main: {
  try {
    node {sh 'false'}
    result = 'success'
  } catch (x) {
    result = "error: ${x}"
  }
}
echo result

This makes sense to me.

(I wonder how this design decision would affect JENKINS-26034, but I'll put my comments about that there.)

I think this is done now.

It's not clear how this is done to me; it seems to still have the undocumented behaviour.

If the retval isn't going to be blessed as a documented feature, it should be explicitly deprecated. "The parallel step's return value should be ignored, and is subject to change in future versions."

Then an example of how results should be reported instead.

Indeed it is still returning a Map so I do not know why csilvers closed this.

I created JENKINS-54514 with proposed documentation changes to address this and related matters. See https://github.com/jenkinsci/workflow-cps-plugin/pull/260 for the patch.

Per this issue, I suggested the return value be ignored. But if it's not considered deprecated I'll tweak that.

In that case this can be considered a duplicate of the broader JENKINS-54514.