What is the meaning of 'SO' in Denali is SO last month?
It's a normal "so" but capitalised for extra emphasis. The writer is saying the Denali name is redundant and oh so extremely old-fashioned.
"So last month" is usually a fashion term that demonstrates how fast the styles change, and that any stylish fashion victim wouldn't be caught dead wearing last month's designs.
In this case, the SO could just as easily have been so.
It's for emphasis.
Like..
This question is so stupid.
Just kidding.