Whistling in Red Dead Redemption (Xbox)? Not a Feature

There is no whistle mechanic in Red Dead Redemption (1 or 2) on any platform.

You may be confusing it with:

• Horse Call (RDR1): Press Up on D-Pad → John whistles / calls horse.
• Horse Call (RDR2): Hold Left Bumper (LB) / L1 → Radial → "Call Horse" (or whistle animation).
• Whistling in other games: The Legend of Zelda: Twilight Princess (Hawk Grass), Skyrim (mod), Assassin's Creed (whistle for horse), Ghost of Tsushima (whistle for horse/guide wind).

In RDR, your horse comes to you via the D-Pad / LB radial. No standalone whistle button.