This depends on the definition of 'supported'
If you buy a product, it will usually include some sort of guarantee of quality and use-worthyness. This would be an obligation that the provider has to give you as part of the terms of the purchase.
Xoops is released under the GPL/ GNU license. There are many implications to this license, but the important facts are stated at the top of scripts as follows
This program is free software; you can redistribute it and/or modify it under the terms of the GNU General Public License as published by the Free Software Foundation; either version 2 of the License, or (at your option) any later on.
You may not change or alter any portion of this comment or credits of supporting developers from this source code or any supporting source code which is considered copyrighted (c) material of the original comment or credit authors.
This program is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.
See the GNU General Public License for more details.
You should have received a copy of the GNU General Public License along with this program; if not, write to the Free Software Foundation, Inc., 59 Temple Place, Suite 330, Boston, MA 02111-1307 USA
So we see here that the Software is Free and NOT warranted or supplied for any specific purpose. This means that there is no official support or communication that you can expect from the authors, developers, suppliers, or distributors.
Ok, None of this is a bad as it sounds, but the official line must be stated.
Although there is no official obligation to the end users of the software, Open source extensively utilises the principle of collaboration and community participation for the development and release of its products.
This means that you will find a very active support network for the core software. The core software referes to the Official Xoops release versions and any other official supporting software. This may include modules, themes and other add on items.
Xoops also encourages third party development of software that works exclusively with the Xoops core system. Software from third party sources may also be supported by the developers or authors, but once again it is a collaborative process without any obligation to the end users.
What all this means in practical terms is that you will find very active communites accross the world of Xoops support websites. They will usually run forums, news items, a downloads section where you can obtain official and maybe third party add-ons like modules, themes and hacks.
Remember this also: This kind of development relies to a large extent on community participation for the purpose of bettering the product. It's a two-way street, you get the support, but you can also collaborate in the commenting and developing process. Xoops has a huge following of community members, many of which are professional software engineers as well as hobyists and casual users. There is an enormous range of skill levels and interests, but overall we all support each other by sharing and exchanging ideas, knowledge and experience.
Ultimately Xoops is a well supported product that relies on collaboration, but not obligation.
For more information on th GNU Free Software foundation click HERE
Also see this FAQ HERE