The light novel ended in 2011, while the Shakugan no Shana TV anime ended in its third season on the same year.
Shakugan no Shana, both the original source light novel and its anime adaptation, have both ended their runs a few years ago.
This would be the first Shakugan no Shana story in four years, however, there’s still no word if this is a one-off story, or there will be a follow-up.
Now, it seems that author Yashichiro Takahashi is reviving the series as Dengeki Bunko magazine’s September 2017 issue included a new story for the franchise.