There are two phases to the game against Mono-Red: dont die and kill them. Your goal is to sideboard into all threats that are good at starting off as dont die and rapidly turn into kill them. Planeswalkers dont do that, and Jadelight Ranger usually just gets sent packing by Abrade or Ahn-Crop Crasher
Also, an honorable mention goes to Abeyance, which doesn't look like much today, but its original wording basically made it a mono-blue Time Walk back in 1997
Legendary Creature - Cyclops Berserker Finisher (CTRL to add secondary) Instant Searing Blaze Searing Blaze deals 1 damage to target player or planeswalker and 1 damage to target creature that player or that planeswalker's controller controls
Gishath, Sun's Avatar, Tana, the Bloodsower, Samut, Voice of Dissent, I could go on but basically a ton of Gruul decks can benefit from Rhythm of the Wild