OS X El Capitan and older not available on App Store?
I am downloading it now from the App Store and it seems to be working fine.  I did not go through the App Store, but used the following link:

https://support.apple.com/en-us/HT206886

After going to the link there is a section on how to get El Capitan from the App Store if you do not meet the system requirements for MacOS Sierra.  Click on that link (pasted below) and it will open the App Store to the El Capitan App download section.  Simply click on 'Get' and it will start downloading.

https://itunes.apple.com/app/os-x-el-capitan/id1147835434?mt=12