so,after i re-read WoK the shin people and szeth's shardblade draw my interest and i started to think that maybe the missing shardblades and plates are in the possesion of the shin. Shinovar is cut from the rest of Roshar because of the Misted Mountains, the shin are considered slow and alien from the other races and almost noone bothers to travel so far (except of merchants) or sees them as a threat, also the shin themselves are a very peacefull race and they would never use the shards to atack others. also taravangian seems to know that the shin gave szeth his blade
so even if we assume that szeth uses an honourblade and not a simple shardblade it was given to him by the shin as far as i understand. we don't know anything about the history of the shin but i believe that some time after the day of recreance people from all over Roshar gathered all the shards they could and retreated in Shinovar so as the world wouldn't be destroyed by the war after the shards. they retread in a naturally well fortified and easily defensible place especially with a lot of shards and to prevent themselves and future generations from using the weapons they created the shin philosophy as it is today. i mean, really, who whould even think to search for shards among the peacfull and rather dull shin?