I got that idea, because I have trouble with my SSH keys. From my point of view, it's a bit strange that you can exactly see, when you added the SSH key, but you can't see, when you used the SSH key last time.
For me as European, I just can see that the key was used on August 8th 2018 02:00 for example.
The scenario I had was the following. I worked on one project and pushed several times my code. Then I created a new one, and at the new one I got an info about permission denied.
I know I had that scenario in a similar form several times as I pushed code and code and code, but I saw in my account settings that the key wasn't really used.