EDIT: This appears to be specific to the PodTemplate "Step" and not the global configuration.

Bug JENKINS-37087 seems to imply that the kubernetes-plugin can support multiple pod labels, resulting in a jenkins slave having multiple labels. 
I cannot seem to find documentation of this and any attempt at adding multiple labels such as "k8s python" for two slave labels fails with error:

ERROR: Labels must follow required specs - https://kubernetes.io/docs/concepts/overview/working-with-objects/labels/#syntax-and-character-set

I've tried defining in a similar method with custom yaml:

podTemplate(label: "python", yaml: """
  apiVersion: v1
  kind: Pod
  metadata:
    labels:
      jenkins/python: "true"
      jenkins/k8s: "true"
  spec:
    containers:
    - command:
      - cat
      tty: true
      image: python:alpine
      name: python
  """

However only the python label shows, not the k8s, when the slave is created.

Is this possible with the current design? The end goal is to match specific CustomTools that require slave labels to run custom install scripts. Otherwise the labels aren't found and the tool cannot install.