In Terminal I cannot type lowercase e
Let's debug it.


Change shells and try again. (Credit to @Kent) In terminal: 


$(which zsh)

Comment out all lines in .bash_profile, .bashrc, etc. and open a new terminal tab/window. If this resolves the issue, something being loaded into the shell environment is consuming the letter e for reasons that science may never be able to explain.
Try cating a file that contains the letter e to see if it will even display: (Credit to @techraf)


Open a text editor (not terminal)
Enter some text with a few es and save file (foo.txt?)
In terminal, cat the file:


cd /path/to/folder; cat foo.txt

If es render then the terminal can handle it, if not, then this is super weird.

Try applescript. (Credit to @ALX)


Open Applescript editor
Create Applescript file with these contents:

delay 10
tell application "System Events" to keystroke "e"

Execute the script file and then quickly navigate to the terminal window. In a few seconds it will virtually press the e key and hopefully show up in your terminal. This would indicate that there could be an input/device driver issue (though I am clueless as to what that might be)



I'm not going to lie, I am absolutely fascinated by this issue and cannot wait to learn what the cause is. It's not hardware because it works in other applications, which means it's software and I cannot imagine who would swallow the letter e with code.