I very much dislike that type of banner and and its location. I find it confusing, if not actually contrary to the message it's trying to convey. The problem is that its interpretation is subjective in many cases.
I personally feel it should be given only a single time, and it should be at the bottom of the question, not at the top. (And, perhaps, it should ...
It was rejected because you mis-defined the 20th century as 1900-1999. The 20th century was actually 1901-2000.
Your proposed tag wiki says:
For questions relating to 20th century English, i.e. 1900-1999.
If you had typed in 1901-2000 instead of 1900-1999, it would have been approved.
Update: status-completed, as I just implemented a new description ...