If you look at the lxde packages (I chose lxde-common) you'll see:
Maintainer: Ubuntu Developers <email@example.com>
Original-Maintainer: Debian LXDE Packaging Team <firstname.lastname@example.org>
Depends: lxsession (>= 0.4.4-2)
Recommends: lxde-core (>= 0.5.0-4ubuntu3)
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.
- 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
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:
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.