summoning horse near the border can get you out of the map and past the border depend on what render distance you using.

/give @p horse_spawn_egg[entity_data={id:"minecraft:horse",EatingHaystack:1b,Tame:1b,Variant:0,body_armor_item:
{id:"minecraft:diamond_horse_armor",count:1},active_effects:[\{id:"minecraft:speed",amplifier:50,duration:99999,show_particles:1b},\{id:"minecraft:resistance",amplifier:255,duration:99999,show_particles:1b},\{id:"minecraft:jump_boost",amplifier:125,duration:99999,show_particles:1b},\{id:"minecraft:absorption",amplifier:1,duration:1,show_particles:0b}],SaddleItem:{id:"minecraft:saddle",Count:1b}}] 1