config.yaml

kind: Widget
metadata:
  name: foo-with-more-than-all-labels
  labels:
    label0: value0
    label1: value1
    label2: value2
    label3: value3
    label4: value4
---
kind: Widget
metadata:
  name: foo-with-all-labels
  labels:
    label1: value1
    label2: value2
    label3: value3
---
kind: Widget
metadata:
  name: foo-with-some-labels
  labels:
    label2: value2
    label3: value3
---
kind: Widget
metadata:
  name: foo-with-none-of-labels
  labels:
    labelX: valueX
    labelY: valueY
---
kind: Widget
metadata:
  name: foo-without-labels

purge-labels.yaml

#@ load("@ytt:overlay", "overlay")

#@overlay/match by=overlay.all, expects="1+"
---
#@overlay/match-child-defaults when=1
metadata:
  labels:
    #@overlay/remove
    label1:
    #@overlay/remove
    label2:
    #@overlay/remove
    label3:
    
#! If "labels" is now empty, remove it entirely
#@overlay/match by=overlay.all, expects="1+"
---
metadata:
  #@overlay/match by=lambda _, left, __: len(left) == 0, when=1
  #@overlay/remove
  labels: