in Gen 1 Swift could never miss, even with Fly/Dig active. Nowadays it misses on foes using Fly/Dig/Dive/Bounce/etc.
Viewing post in The Legendary Pokemon Breeder comments
I don't understand what you're getting at? Swift, alongside Faint Attack (Dark, Physical), Shadow Punch (Ghost, Physical), Aerial Ace (Flying, Physical), Magnet Bomb (Steel Physical), Smart Strike (Steel, Physical), Magical Leaf (Grass, Special) and Shock Wave (Electric, Special) bypass accuracy checks, which means user's accuracy and target's evasion are not factored in when determining if it hits.
These checks are also only considered for targets in the semi-invulnerable state from moves like Fly, Dive, Dig, Bounce, Sky Drop, etc. if they are targetted by a move that can hit them (Thunder, Twister, Hurricane, Sky Uppercut for Fly/Sky Drop/Bounce. Surf for Dive, Earthquake/Magnitude/Fissure for Dig) None of those moves are among the Swift varients.
Oh Forgot the Grimmsnarl-line's signature move False Surrender, that's also among the Swift varients.
(For some clarity I have been playing the games since gen 1.)