@imakin I would say that the wiki files have not been added because nobody contributed to doing it, there was an attempt to make SCSS a first class filetype waaaay back in #20 and then a custom filetype #260 (which is pretty much the same as the wiki entry you linked), but nobody contributed to them so they didn't go anywhere, and now have bitrotted and have conflicts.
As I explained to @martonlente on #3229, everything in Geany is done by contributors, so if nobody contributes it, it won't happen.