It is very important to keep your Qubes OS system up-to-date to ensure you have the latest [security] updates, as well as the latest non-security enhancements and bug fixes.
For example, you might be using a [Fedora TemplateVM].
The [Fedora Project] is independent of the Qubes OS Project.
They set their own [schedule] for when each Fedora release reaches [end-of-life] (EOL).
You can always find out when an operating system reaches EOL from the upstream project that maintains it, but we also make EOL [announcements] and publish guides for official TemplateVM operating systems as a convenience to Qubes users.
When this happens, you should make sure to follow the guide to upgrade to a supported version of that operating system (see the [Fedora upgrade guides] and the [Debian upgrade guides]).
The one exception is dom0, which [doesn't have to be upgraded][dom0-eol].