I can see only:

but in http://mirror.centos.org/centos/8-stream/storage/x86_64/

there's only Ceph Pacific.

Looks like https://koji.mbox.centos.org/koji/buildinfo?buildID=16908 is missing there.

Installing it by hand: yum install https://koji.mbox.centos.org/pkgs/packages/centos-release-ceph-pacific/1.0/1.el8/noarch/centos-release-ceph-pacific-1.0-1.el8.noarch.rpm

I see repo is using $release instead of $stream so looks like it will pick up the wrong repo.