If one chooses "Upstream build which trigger" as option for copying artifacts there is only the possibility to get "last successful as fallback" and not "last unstable" which I would like to have as well.
However the wording for last successful is inconsistent in my opinion because if you just choose copying from "Lastest succesfull build" also unstable builds are considered.
If you look at
JENKINS-11828 and the merge of pull request #7 you see the place which my feature could be implemented. (see return isFallbackToLastSucessful() && run.getResult().isBetterOrEqualTo(Result.SUCCESS);