Pressing first letter of user account name does not select user in login window This has been irritating me for a long time and I have not found a solution for this whenever I have looked for one. 

This problem has perplexed me maybe as far back as Mac OS X 10.7, but I can't be sure. We share a couple of Macs in our family of five and because of that we each have dedicated user accounts. In total that's five separate user accounts. After booting up we get the login window just fine. However, the only way to select the user account you wish to log in to is to click on the user's name. In the past we could just press the first letter of our username and that would automatically select that user for you. But this hasn't happened for a long time now. 

The reason why I think it may not have occurred since Mac OS 10.7 is that one of our Macs is still on Mac OS X 10.6.8 and on that particular Mac we can still press the first letter of our username to select that account. The Mac this doesn't work on is an iMac using Mac OS X 10.11.6, but like I said, this has been happening for a long time, even before Mac OS X 10.11. It's not the end of the world, but it's surprising how annoying this is for us. 

MY QUESTIONS:


Did Apple remove this feature for some reason?
If not, is there a setting anywhere to switch this feature back on?
Did Apple remove this feature for some reason?

No, this has not been removed. You should still be able to select users at the login window via the first letter of their user name, or via using the tab and arrow keys.

If not, is there a setting anywhere to switch this feature back on? 

At the outset let me say this may not be the likely solution for you. However, I have seen this behaviour and thought it was worth mentioning in case it helps you or others.

If your Mac running El Capitan is a desktop Mac (i.e. iMac, Mac Mini, Mac Pro, etc) and it happens to be plugged into a UPS (i.e. an Uninterruptible Power Supply), then shut down your Mac and unplug it from the UPS. Now plug it into another AC power outlet and reboot it. Check to see how your login window behaves now?

If on the off chance this is your situation and your Mac now works as you'd like, an option that may help you is upgrading your OS to macOS Sierra. I say this only because macOS Sierra offers some native support for using a UPS and this may or may not rectify this issue.