Since this is a new way of providing LTS, by allowing each flavor to decidee the length of support, all the details may have to wait three years. However, under the 3/5 year support plan, anything Xfce specific will no longer be updated or fixed after three years. Also, bug reports will no longer be accepted after three years.
I do not know if the developers will attempt to determine the desktop package, and only allow certain systems to have updates, or if some updates will be allowed for everyone for five years.
If you look at the lxde packages (I chose lxde-common) you'll see:
Package: lxde-common
Priority: optional
Section: universe/x11
Installed-Size: 1124
Maintainer: Ubuntu Developers <ubuntu-devel-discuss@lists.ubuntu.com>
Original-Maintainer: Debian LXDE Packaging Team <pkg-lxde-maintainers@lists.alioth.debian.org>
Architecture: all
Version: 0.5.0-4ubuntu3
Depends: lxsession (>= 0.4.4-2)
Recommends: lxde-core (>= 0.5.0-4ubuntu3)
Suggests: lxlauncher
Filename: pool/universe/l/lxde-common/lxde-common_0.5.0-4ubuntu3_all.deb
Size: 832896
MD5sum: 800d4f0c55534081334405c74abd2421
SHA1: ad0a5798080e3fbc95b1a039811b1c48b5a8eb41
SHA256: 1e0cd437d89948ada2a02f1360dfa15d0a674e6a9e9e61d87270ac12d7fb15e9
Description-en: the Lightweight X11 Desktop Environment configuration data
LXDE (the Lightweight X11 Desktop Environment) is a project aimed
to provide a new desktop environment which is lightweight and fast.
.
It's not designed to be powerful and bloated, but to be usable and slim
enough, and keep the resource usage low. Different from other desktop
environments, lxde don't tightly integrate every component. Instead,
lxde tried to make all components independent, and each of them can be
used independently with fewer dependencies.
.
Features:
- Lightweight, runs with reasonable memory usage
- Fast, rund well even on older machines produced in 1999
- Good-looking, GTK+ 2 internationalized user interface
- Easy-to-use, the user interface is simple, but usable enough
- Desktop independent (suprise! Every component can be used without LXDE)
- Standard compliant, follows the specs on freedesktop.org
Homepage: http://lxde.org
Description-md5: c6b780702af08f256b4707025f34cc99
Bugs: https://bugs.launchpad.net/ubuntu/+filebug
Origin: Ubuntu
The hint you're looking for is the Section: universe/x11. This tells you the package is in Universe:
This means that it's community-maintained, and since the people who maintain LXDE announced that they won't be supporting it then basically you can't be guaranteed that it will be supported for as long as you need it.
If you do an apt-cache show unity you'll see a line that says:
Supported: 5y
This means that the package is supported by Canonical for the life of the LTS.
Things in universe, on the other hand, it depends on community best-effort, there's something like 20k packages in universe, so there's no way of really telling which ones the community will be interested in maintaining and which ones will be ignored. If Lubuntu has said they're not planning on supporting it for 5 years then it's likely they mean those packages as well (though I can't speak for their project).
PCmanFM and openbox are also in universe, so the same thing probably applies. Sorry there isn't a real solid answer, but it's hard to make guarantees on what will be supported when it's based on people's volunteer time.
Best Answer
The core aspects of Ubuntu get full support for the full period. That includes kernels, pre-installed applications. You should be fine with Firefox updates. It's the XFCE stuff that will start to lag behind.
The repositories for Xubuntu and Ubuntu are the same — it's just a different selection of packages. If you want full Xubuntu support indefinitely you either need to upgrade in a timely manner or maintain (or pay somebody else to maintain) all the packages you need indefinitely.