Just upgraded to Foreman/Katello 3.16/4.18. and looking in /etc/yum.repos.d/foreman.repo I see:
[candlepin-source]
name=Katello Candlepin source
baseurl=https://yum.theforeman.org/candlepin/4.18/el9/source/
gpgkey=file:///etc/pki/rpm-gpg/RPM-GPG-KEY-candlepin
enabled=0
gpgcheck=1
Looks like a typo here. I assume it should be:
baseurl=https://yum.theforeman.org/candlepin/4.6/el9/source/