This is a terrible idea because it's blacklist-based security. Places like GitHub have their own ssh server that only runs the git server and nothing else.
It is a common username (see: https://en.wikipedia.org/wiki/Toor_(Unix) ), the machine I am typing this on has it.
If we are going to be clever we should follow the statistics and go for 12 that where not on the toplist!!
12 - Rank 318
21 - Rank 523
I'm surprised that '12' is so low on the list.