What it does
Long Reach strips the contact flag from every move the user makes, regardless of whether the move normally makes contact. Any effect that checks for contact — Rough Skin and Iron Barbs recoil, the chance-based triggering of Static and Flame Body, King's Shield's Attack drop penalty, Gooey's Speed drop — none of them fire against a Long Reach user. Decidueye using Brave Bird won't trigger the opponent's Rough Skin, won't set off Iron Barbs, and won't lose Attack from a King's Shield.
Who runs it
Decidueye is the primary carrier at 80% usage. Its Grass/Ghost typing gives it both a Ghost STAB option and solid Grass coverage. Brave Bird is its hardest-hitting move but normally carries a contact flag, which means Rough Skin/Iron Barbs users would chip it down fast. Long Reach removes that problem entirely. The same applies to Leaf Blade and Spirit Shackle.
How to use it
The main payoff is using Brave Bird freely into Iron Barbs and Rough Skin matchups. Without Long Reach, hitting an Iron Barbs target twice with Brave Bird can shave off a dangerous chunk of HP on top of the move's own recoil. With Long Reach, that entire consideration disappears. Against Flame Body users, physical contact no longer risks an accidental Burn. Spirit Shackle is a key doubles utility move since it prevents switching — Long Reach means using it carries no contact-related downside.
Tips & strategy
The matchup where Long Reach is most impactful is against King's Shield users. Aegislash's King's Shield inflicts a -2 Attack drop on contact move users, but Decidueye ignores this entirely, letting you throw Brave Bird or Leaf Blade into a King's Shield without losing Attack. One thing to be clear about: Long Reach removes the contact flag, but it does NOT remove Brave Bird's own recoil damage — that recoil comes from the move mechanics itself, not from contact triggering. You still take recoil from Brave Bird. What you don't take is extra damage or penalties from the opponent's contact-triggered effects.