I know that in the past if I've previously downloaded OS X El Capitan from the App Store I could download it again from the Purchased tab in the App Store, however now that macOS Sierra is out I'm not able to download it as it says INSTALLED (and grayed out) even on a system running OS X Mountain Lion 10.8.5. Pressing any key combination, e.g control, option, etc. doesn't change the button to DOWNLOAD. All other previous versions of OS X can still be downloaded from my Purchased tab in App Store as they still say DOWNLOAD and are not grayed out, thus active.
The image below is from a system running OS X Mountain Lion 10.8.5, does not have OS X El Capitan installed on it or even the Install OS X El Capitan.app on its filesystem, yet it says it's installed and grayed out.
Best Answer
Whether or not you've downloaded it in the past, you can now download OS X El Capitan 10.11.6 (6.21 GB) via this link which will open first in the Browser, then in the App Store.
Note: If doing this to downgrade from macOS Sierra.
In order to use the Install OS X El Capitan.app from macOS Sierra you need to create a USB Installer and boot the Mac with it. You'll also need to erase the Macintosh HD and perform a clean install. Make sure you have proper User Data Backups from which to restore your Data before erasing the Macintosh HD.
Have a look at Create a bootable installer for OS X. Excerpt show below: