Critical (imo):

• The expand/collapse button needs a focus state (covered by Improve a11y in EuiNavDrawer #2417)
• On clicking the expand/collapse button, focus management is lost and is returned to the body. (It should stay on the button.) (EuiNavDrawer a11y, pt. 1 #2643)
• The navigation docking button sometimes has a focus state but also it blends into the hover state of the expand/collapse button so it's difficult for me to tell what's really going on. (covered by Improve a11y in EuiNavDrawer #2417)
• All of the "pin to top" buttons have the same label so in a rotor they're indistinguishable. Should be changed to something like: "Pin {link name} to top". (covered by Improve a11y in EuiNavDrawer #2417)
• Should be wrapped in a nav element (covered by Improve a11y in EuiNavDrawer #2417)
• Tab order in the nav list is backwards of what it should be. It starts from the bottom of the nav and moves it's way up. Likely it should be the opposite.
• Clicking on a grouped item should move focus to the inner menu, likely similar to trapped focus in popovers. Right now it just exposes the menu, but you need to cycle through the other items to get to them. (EuiNavDrawer focus management, flyout auto-close #2749)
• Group buttons need to announce themselves as much. Right now they are just listed as a button. (EuiNavDrawer focus management, flyout auto-close #2749)

Less so:

• The dock/undock button doesn't show up in a screen reader's rotor when the nav is expanded. (Should work the same as the "pin to top" buttons which do show up.)
• All of the "pin to top" buttons should have aria-pressed=true|false defined on them to communicate their state.
• The dock/undock button should have aria-pressed=true|false defined to communicate state. This can replace the role=switch and aria-checked=true|false.(EuiNavDrawer a11y, pt. 1 #2643)

Up for debate:
1. I think the focus order would be better in visual order. I get the argument that it's to keep users from having to tab through all the options, then hit "expand", then potentially go back to the option they want but I think it's so unexpected that it's difficult to find and you lose your place instead of it feeling smooth. I tried playing around with a super obvious focus state and that grabbed my attention enough that I knew what was going on but that'd be pretty far away from our current focus state styles.
2. I think EUI should provide a skip link past the navigation.