There are many spells (not only from Castle Nathria) that return 404. I figure this out by going through the new journal-encounter endpoints. I think this happens on many bosses from dungeon and raids.
Here are a few examples:
Incineration from Nyalotha
/data/wow/spell/306111?namespace=static-9.0.1_36072-eu
also spells from the new raid
/data/wow/spell/329362?namespace=static-9.0.1_36072-eu
/data/wow/spell/328857?namespace=static-9.0.1_36072-eu
All of these return 404.
Would be nice if I could fetch these informations before SL.