We have Jenkins masters on Ubuntu 20.04. They started showing end-of-life monitor alerts yesterday. According to the data in https://github.com/MarkEWaite/jenkins/blob/master/core/src/main/resources/jenkins/monitor/OperatingSystemEndOfLifeAdminMonitor/end-of-life-data.json#L131-L132 that is generally correct, as it's exactly 6 months from now.

However, the alert being shown doesn't use the date from the JSON file, but rather the default date from the Java code (the field initialization value at https://github.com/MarkEWaite/jenkins/blob/master/core/src/main/java/jenkins/monitor/OperatingSystemEndOfLifeAdminMonitor.java#L76)