As per the instructions here, https://pkg.jenkins.io/redhat-stable/, it has you download the https://pkg.jenkins.io/redhat-stable/jenkins.repo file. This file's header line is "[jenkins]". This causes yum to think that it should be grabbing the latest "jenkins" mirror version, which isn't available in the "jenkins-stable" mirror.
Change the first line in the downloaded "/etc/yum.repos.d/jenkins.repo" file to read:
The output of attempting to install jenkins after using this repo file is below:
http://pkg.jenkins.io/redhat-stable/jenkins-2.89-1.1.noarch.rpm: [Errno 14] PYCURL ERROR 22 - "The requested URL returned error: 404 Not Found"
Trying other mirror.
Error downloading packages:
jenkins-2.89-1.1.noarch: failure: jenkins-2.89-1.1.noarch.rpm from jenkins: [Errno 256] No more mirrors to try.