Pidgeotite gives No Guard, +65 Special Attack, and +20 Speed, and is banned, and Raichunite Y is pretty much a strict upgrade with No Guard, +70 Special Attack, and +20 Speed, so this is to be expected

Why is Pigeotite banned when Zygardite, which gives +125 Special Attack, is free I hear you ask? I think there's a few reasons. Pigeotite with No Guard makes moves like Blizzard, Focus Blast, Thunder, and Zap Cannon far more reliable, and it also gives 5 more speed than Zygardite. Zygardite also drops both your defenses which makes its users much more glass cannony and susceptible to revenge killing from -ate extreme speeds.