The message preferences page, found at Site Administration > Preferences > User Account > Message Preferences, is not accessible in so many ways:

Let me count some of the ways (off the top of my head):

1. As someone who can see, I can't read the rest of the page because a popup automatically appeared over half the page.
2. The appearance and the way that the page works is unexpected and disorienting. This works so differently than 99.999% of all the pages in Moodle.
3. For most people, if they have never seen messages overview before, they won't even know what that is or what to do with it. They will likely just leave the page but may not be able to do so because the popup is covering almost everything.
4. The < arrow is misleading. I expected something like this to expand the popup window. I had to read the HTML to find out that it is a link to go back to messages overview (which is not where I came from):
   
5. If I use keyboard navigation, some of the links on the page are under the popup window.
6. On anything larger than a mobile screen, the X to close the window is missing so they can't close the popup to get to the content it is covering, only change its content to messaging which has nothing to do with messaging preferences.
7. If you do manage to close the popup window, you will discover an H2 on the page with no content.
8. If Messaging System is disabled on the Moodle site, this preference page also results in an H2 with no additional content.
9. On wider screens, users of screen magnifier will never even notice the popup because there is no indicator that a popup showed up all the way at the other end of the the screen.

I believe it would be beneficial to ensure that the UI remain consistent with the rest of Moodle. I don't go into this page very often but I think it is important to ensure that is accessible to everyone.