the rules say that SNES Classic is banned but WR for Kefka at Narshe is on SNES Classic ?

It saves several seconds over the original hardware because it doesn't handle lag correctly, so yes, it is banned. That was decided after the current WR was established. They haven't disqualified the WR run because of it, though.

I spent the whole day learning Kefka@narshe playing on SNESClassic. Can I submit a run on SNESClassic or will it get rejected?

(Classic is listed on the left above the follow/unfollow button and ffmonks run is on Classic)