Bleach Wiki works on the premise of going directly off what's said in the manga. In chapter 414, page 4, Gin said he killed Rangiku. At Bleach wiki, we have to take his word for it and declare her dead. However, that does not exclude the possibility that Gin was lying or that Orihime, who is capable of bring back the dead, won't resurrect Rangiku in the future. Simply that until we are told otherwise, we have to take Gin's word for it and according to Gin, he killed Rangiku.
Following the release of chapter 416, it was established that Rangiku was not dead that so the wiki was changed to reflect this.