noted in https://github.com/jenkinsci/jenkins/pull/6193#discussion_r788853559

if a job label expression such as "foo || bar"  gets added to jenkins.labels that label expression is never going to be garbage collected as long as one such cloud exists that can provision foo or bar or there is a node tied with either of those labels.

even the job or reason for that label expression to exist is removed and the expression is no used anywhere else this expression appears to remain.