An administrator received an email stating that the OS they are currently supporting will no...

When an OS is no longer receiving updates, it is generally because it has reachedend of life (EOL), meaning the vendor has ceased support.

FromQuentin Docter – CompTIA A+ Complete Study Guide:

“End-of-life refers to the point when an OS vendor no longer issues updates or support. Systems at EOL pose security risks and must be upgraded or isolated.”