The operating system end of life monitor shows you the date when the operating system will no longer be supported by the Jenkins project.
It should state that Ubuntu 23.04 will not be supported by the Jenkins project after January 20, 2024, because that is the end of life for Ubuntu 23.04 according to the Ubuntu releases wiki page and the end of life date site.
The warning is visible to you because it is now less than 6 months before the end of life of Ubuntu 23.04.
We don’t test non-LTS versions of Ubuntu, but I’m not aware of any reason they would have any issue that is not also visible in LTS versions of Ubuntu.