All discontinued items in ORAS:
RBY Items:
Oak's parcel
Card Key
Lift Key
Bike Voucher
Silph Scope
Secret Key
Gold Teeth
Coin
Coin Case
X Special
Itemfinder (now Dowsing Machine)
Safari Ball
Pokeflute
GSC Items:
Apricorns
All apricorn balls
Basement Key
Blue Card
Lost item
Machine Part
Mystery Egg
Pass
Rainbow Wing
Silver Wing
Red Scale
Exp. Share
Slowpoketail
Squirtbottle
Gold Leaf
Gorgeous Box
Brick Piece
Normal Box
Silver Leaf
All the GS berries
Berserk Gene
Polkadot Bow
DPPt Items:
Azure Flute
Fashion Case
Coupon 1
Coupon 2
Coupon 3
Explorer Kit
Galactic Key
Journal
Lunar Wing
Member Card
Old Charm
Pal Pad
Parcel
Poffin Case
Point Card
Poke Radar
Seal Case
Secret Potion
Sprayduck (now Wailmer Pail)
Suite Key
Works Key
BW/B2W2 Items:
All gems except Normal Gem
All wings except Pretty Wing
Relic items
Dark Stone
Gram 1
Gram 2
Gram 3
Dragon Skull
Light Stone
Liberty Pass
Lock Capsule
Prop Case
Xtransceiver
Colress MCHN
Dropped Item
Grubby Hanky
Plamsa Card
XY Items:
Adventure Rules
Holo Caster
Honor Of Kalos
Elevator Key
Power Plant Pass
Prof's Letter
Looker Ticket
Lens Case
Roller Skates
Sprinklotad
TMV Pass
All this and the different kinds of Mails, exclusive to their regions are also discontinued, except for those of RSE origin. No items from RSE were omitted, as it's ORAS's parent game, and the story line is almost exactly the same.
Hope I helped!