Installing Windows XP via Bootcamp no longer supported? I have a Mac Mini Early 2009. I'm on El Capitan. According to apple's compatibility table, here Windows XP is supported in Boot camp , provided you use your original OSX DVDs to locate the drivers.

However, it would appear that the only option offered in boot camp assistant is to install Windows 7. My assumption is that Windows XP can only be installed via boot camp if I have a much older OSX version installed, 10.6. Or, if by chance the boot camp assistant from 10.6 (3.x) can be installed in El Capitan. 

The documentation is misleading as it does not specify the starting point I need to be at to install Windows XP via bootcamp, only that I need the original OSX DVD to get started...
I will try to answer your question as best that I can. It is a question of timing. By 2009, Apple was shipping Macs with firmware or offered firmware upgrades that allowed Windows to be installed in BIOS/MBR mode. This firmware is stored in your Mac. The firmware does not change when you upgrade to a new version of OS X. Therefore, the version of OS X currently installed has nothing to do with whether you can run Windows on your Mac. In fact, it is possible to run Windows on your Mac without any version of OS X installed.

To install a particular version of Windows on a Mac, a few timing aspects need to be considered. The Windows installation DVD needs recognize your Macs hardware in order to install the drivers needed to install and boot to Windows. Since Windows XP was released in 2001, one should not expect the DVD (or maybe CD) to contain the drivers for the 2009 Mac models. Granted, to a certain extent, hardware can be designed to operate in a legacy mode in order to install an older operating system. When the difference is 8 years, this is difficult to do. Also, Microsoft installations allow for newer drivers to be introduced during the installation process, but this was never employed for Windows installations on Macs in 2009. Instead, Microsoft released Windows XP DVDs with Service Packs already installed. These newer DVDs contained the drivers need to allow Windows to install and boot on Macs. Once, Windows was up and running, better drivers could be installed to upgrade existing drivers and support other hardware such as the camera and sound. These drivers were part of a package referred to as "The Boot Camp Support Software". This software also provided a way to boot back to OSX and a utility to update Apple software such as iTunes. When "The Boot Camp Support Software" installs, the Windows partition is renamed BOOTCAMP. For this reason, it has become customary to refer to the Windows installation on a Mac as Boot Camp. In fact, you can rename the partition to anything after installing "The Boot Camp Support Software".

Apple's web site says you can install Windows XP on your Mac. Whether you need SP2 or SP3 included on the DVD is just a matter of timing. I do not have an answer. SP2 was released in 2004 and SP3 can out in 2008. Since SP3 was the last offered for XP, such a DVD should work for you.

Although your question did not ask about installing a newer version of Windows, I will include my thoughts anyway. Generally, Microsoft does not require new drivers for each release of Windows. For example, when upgrading Windows, the newer version just adopts the drivers from the previous version. Also, Microsoft may offer new drivers though Windows Update. Windows 10 appears to have he ability to connect to the internet and download drivers during the installation process. 

Apple's web site reports your Mac can only run 32 bit XP, Vista and Windows 7. This probably is not true. You have a 2.0 GHZ Core 2 Duo 2.0 processor. I am writing this answer on a 2007 iMac with a 2.4 GHz Core 2 Duo processor. The operating systems is Windows 10 Pro 64 bit. Apple is not entirely wrong. The Boot Camp Assistant will not let you install the newer Windows operating systems, but you don't have to use the Boot Camp Assistant. I guess your complaint is the Boot Camp Assistant version that comes with El Captain OS X 10.11 will not let you install the older XP operating system. Again, the Boot Camp Assistant is just an assistant used to make windows easier to install. It does not have to be used.