As you note, the legacy Saffire Pro 26 I/O (not the current Pro 26 model) has been discontinued for quite some time. It uses Apple’s FireWire Audio driver which is a built in part of OS X, Apple announced that they would no longer be updating this will the release of El Capitan.
The Pro 26 I/O has not been officially tested with El Capitan, however I would still expect it to function as an audio device (though this is subject to change at any point due to Apple no longer updating the FW driver). While it may function as an audio device, the Saffire Control routing software will likely not run as smoothly as on supported operating systems and may well be ‘laggy’. If you do choose to update, I would also strongly recommend not running the firmware updater for the Pro 26 I/O on an unsupported operating system.
All of our current Saffire models, as well as the USB Scarlett range and Thunderbolt Clarett range are fully supported on El Capitan in case you’re considering an upgrade to the Pro 26 I/O: http://focusrite.com/products
I hope this helps, if you have any further questions feel free to post or you can contact us directly here: http://focusrite.com/answerbase/contact-0
Jack // Focusrite Technical Support