I don't think there's much reason for it since it's generally pretty obvious what items are good and what items aren't especially in competitive. Showdown by default shows a list of items by five categories anyways:
Popular Items: Items frequently seen on numerous Pokemon, contains typical standbys like the health restoring berries, choice items, leftovers, etc.
Items: A long list of items with useful effects, but can't easily be as splashed on like the Popular items. The list is too long but notable items include Z-Crystals, most berries, plates, just anything with a useful side effect.
Pokemon-Specific Items: This category is exclusively for items meant only for certain Pokemon, as well as Mega Stones (though specific crystals aren't here for some reason)
Usually Useless Items: These items are typically outclassed by other choices (status berries, just use Lum), don't do much of note (when was the last time you saw the Destiny Knot activate?), or hinder you in the process, like the Iron Ball. Items like these have an additional effect, no matter how situational or useless.
Useless Items: A long, thorough list of items that do nothing at all are are only usable as Fling or Natural Gift. These also include the type gems, because only the Normal Gem has ever been released in Gen 6 or Gen 7.