The short answer is yes. The slightly longer answer is, yes, but no two letter domains remain. Can you register something like d3.io? Yes. Can you register something like dx.io? No.
[1] I posted a list of letter + number .io domains recently: https://news.ycombinator.com/item?id=5672237 - most are still available, but some were immediately taken due to the post and due to FramesPerSushi's tool (which I found rather useful).