ocp 4.15 was GA-ed, but we cant upgrade to it, it is missing from all known channels I've checked including candidate-4.15:

Channel: stable-4.14, candidate-4.14, candidate-4.15, eus-4.14, fast-4.14, fast-4.15, stable-4.14

    [kni@ocp-edge119 ~]$ oc version
Client Version: 4.14.0
Kustomize Version: v5.0.1
Server Version: 4.14.0
Kubernetes Version: v1.27.6+f67aeb3

 100%   

   1.deploy a 4.14.0 ga version, I was using this job:
https://auto-jenkins-csb-kniqe.apps.ocp-c1.prod.psi.redhat.com/job/ocp-baremetal-ipi-deployment/28608/parameters/

   2. choose a channel from the list of available channels (candidate-4.14, candidate-4.15, eus-4.14, fast-4.14, fast-4.15, stable-4.14)
[kni@ocp-edge119 ~]$ oc adm upgrade channel stable-4.14
    3. check the recommended upgrade versions list: 
    [kni@ocp-edge119 ~]$ oc adm upgrade --include_not_recommended

- no 4.15.0 available,
- I did see an old version of 4.15.0-ec.2 at candidate-4.15 channle

    4.15.0 is suggested