This is a question, which cannot be answered.
CUCM 15 isn't out yet and no documentation about it too (at least I didn't find something).
But what is the intention of your questions?
Cisco just uses CentOS as a base OS, but it is not the original OS anymore, that you can download. Any end of x notifications of CentOS have nothing to the with the CUCM OS.
I do recall hearing something about this at either Cisco Live 2022 or it could have been a session in CCP, Cisco Customer Connection Program, but I can’t remember what they specifically said. All I do remember is that there will be a new OS from version 15, but for the life of me I can’t recall what it will be. If it was in CCP it would be under NDA, so even if it could be found there we would not be allowed to use that in a public community nor in communication with any customers.
Just heard that the replacement OS is called Alta. I found out because our Cisco liason is trying to get on our calendar to "discuss timelines", which I've never had happen with any other release of CUCM. That tells me an upgrade to version 15 is going to be very complicated. Its easy to go from Redhat to CentOS. Its not really easy to go from CentOS to anything else.
As understood from the live session, the client who has CallManager version 14.X needs to be ready for the mandatory migration to CallManager's new release version 15.
And this is because CallManager version 14 is built based on the Redhat contos 7 which will be out of support soon at the end of 2024.