Is there any logical explanation why we are still prohibiting shard? It's been half a decade since Arena came out. For all practical purposes, shards are approximately equally difficult to acquire as the rest of the rares. If the answer is that shards are too powerful, then simply restrict the most powerful ones, just like what you do with rare weapons.
If there's anything we should do a blanket ban of, it's Nymphs, which are actually difficult to acquire.