Cfrgtkky

We are using Ubuntu Server 18.04, and is supposed to have 5 years of support, but we also need a graphical GUI and we want to install Xubuntu packages.

Since Xubuntu 18.04 only has 3 years of support, what's going to happen with this server after those 3 years?. Are we going to face security issues? All the server is "downgraded" to only 3 years of support?.

Ubuntu Server is not intended for use with a Graphical Interface.

When the time comes and the GUI you have installed ends the support, you won't receive any updates, new features or bug fixes for that GUI.

It might not be of your interest about new features, but bug fixes are almost identical to security fixes. So, from this perspective yes you will have a problematic server running.

Also, I suggest to read this answer as describes why Ubuntu Server developers do not recommend installing X on a server.

I'm also quoting here.

Most Ubuntu Server developers do not recommend installing X on a
server. There are multiple reasons for not installing a GUI.

Some reasons to not install a GUI include:

1. You'll have more code subject to security vulnerabilities, more packages that need updating, and more server downtime.


2. X11 and desktop packages are not supported for the full 5 year lifecycle of the LTS server release.


3. Performance may suffer because resources (memory, hard disk space, CPU, etc.) will be consumed by the GUI.


4. It is best practice to only install needed software on a production server.


5. The GUI may include other network services that are inappropriate for a server.


6. One of the goals of Ubuntu Desktop Edition is to make it easier for users to use Linux. When installing some desktop environments,
   services that you may not specifically want will be installed. For
   example avahi-daemon, which is used to help configure networking,
   adds another open port and may introduce unwanted DNS conflicts with a
   .local domain.

So for the most secure server it is best to not install a GUI.

To the best of my knowledge Ubuntu LTS versions such as 18.04 have the same 5 years of support since Ubuntu v.12.04 whether you've installed a desktop version or a server version. The support schedule prior to 12.04 was 5 years for server and 3 for desktop. Of course any packages that you choose to install from PPA's you do so at your own risk.

So, no I don't see any reason to expect that your server will be downgraded to 3 years of support. Of course if you've purchased a support contract somewhere, it's always best to review the terms of any contract to insure compliance.

To obtain a list of packages that are reported to be supported you can issue the command:

ubuntu-support-status --show-supported

To obtain a list of your packages that are reported to be unsupported you can issue the command:

ubuntu-support-status --show-unsupported

Note that just because a package is listed as unsupported doesn't mean you won't be able to find someone to help you with it here, or on another Stack Exchange site.