Original description:
The letters are lowercase in the controls menu even though they are suppose to be capital letters by default.

The "Not Bound" is missing if you turned off the selected control that you do not want to be active.

This can also apply when you switch from an older version to a new version and it greets you to move around with W A S D keys.

@Bentroen has a description from below which explains how this happens. (Added in by @Oval)

From MC-142454 by Bentroen:
In the Controls screen, the key names for the letter keys used to be uppercase letters both before the change to LWJGL 3, which revamped key names (versions prior to 1.13), as well as during most of the 1.13 development cycle. Though, starting in 1.13-pre4, for some reason (probably another update to LWJGL) they're not uppercase anymore.

The reason for this report is that it's quite unusual to refer to single-letter key names in lowercase. As an example, the basic controls for first-person games are WASD, not wasd; the most widely used keyboard layout throughout the world is QWERTY, not qwerty. It just looks plain weird and is inconsistent with the other key names, which are all properly capitalized (Space, Tab, Left Shift etc.).

The reason for this might be that Minecraft is simply using a character, not a key name in itself, to display what key that character is assigned to. When the user presses the 'A' key, the game listens to that input and receives 'a', then simply uses the character 'a' to display that key (not caring whether it is capitalized or not). Since keys can also be mapped to non-ASCII keys (such as 'Ç'), using language strings for that wouldn't work, as that would result in a lot of entries.

This also affects other places where key names are shown, e.g. the tutorial at the beginning of the demo mode. See screenshots.