Is 未長未短 grammatically correct (in classical Chinese)?
The short answer is yes, especially in written. It's kind of “文言文” so may hardly be seen in oral speaking.


  游览路线也依旧如故，未长未短


Here “未长未短” means "it is neither longer nor shorter than before", in other words, "its (the route's) length is not changed".

Let's read the whole sentence again and you will be clearer.


  但与大门前的“翻天覆地”相比，……，景观还是两年前的样子，没多没少，游览路线也依旧如故，未长未短


“未长未短” actually act the same role as “没多没少”, both of them means "things not changed". Conversely, “翻天覆地” means "things changed significantly".


  如意金箍棒复得, 未长未短


Great sentence if you want to say 金箍棒完好无损。