what is label shift?
Label shift is the opposite of a covariate shift.
 In this case, the assumption is that even though the feature distribution remains the same, the Label distribution might changes.
 e.g. Symptoms --&gt; Diseases
 It can be different for different country (based on medical education of the Country/Doctor)
 It can change with time also based on advancement in Medical knowledge
  Similar logic can be built for &quot;Words --&gt; Slang&quot;. It can change with time due to the acceptance of these words.
Read the references for a formal explanation.
References-
Dive into Deep Learning
Detecting and Correcting for Label Shift with Black Box Predictors