Ah, that's something else indeed. I guess it does show up in the TODO list which is the button to the right of the one I linked earlier, but there's a lot of other cruft in there, including already merged MRs.
The ToDos is a sort of inbox of actionables. We don't use approvers because we use assignees for MRs but here you'll get anything that requires (or may require) some action on your part, like mentions, assignments, CI job failures you're responsible of... ToDos get automatically "done" when you act, such as replying to a mention. Alternatively you can manually mark some issues as "done" or "to do". I like this workflow personally use them to great effect by practicing a form of Inbox Zero, and combined with the personal issues and MR lists they give me a good picture of what I have to do.
In other words, there's a number of issues assigned to me listed there, even though they're closed. There's also a few mentions that I don't otherwise interact with (I've seen them in my email and that's that), so I'd have to manually mark those as Done in the GitLab interface after reading them in my email.
We completely agree with you here. We want TODO to be meaningful and useful to everyone. The UX team is working through an exploration issue to address this, we would love to hear your thoughts on the issue: https://gitlab.com/gitlab-org/gitlab-ce/issues/43474
