Hero Capes

You can use CurseMaven to add Hero Capes to your project as a dependency.

You don't need to touch anything client-side. You just need to cast the player to HeroCapesTracker and set your custom ResourceLocation. Make sure to set it back to null when you don't use it.

@SubscribeEvent
    public static void onServerTick(TickEvent.ServerTickEvent event) {
        if (event.phase == TickEvent.Phase.END) {
            if (ServerLifecycleHooks.getCurrentServer() != null) {
                for (ServerPlayer player : ServerLifecycleHooks.getCurrentServer().getPlayerList().getPlayers()) {
                    HeroCapesTracker tracker = (HeroCapesTracker) player;

                    if (player.getItemBySlot(EquipmentSlot.CHEST).is(Items.IRON_CHESTPLATE)) {
                        //tracker.herocapes$setCapeEnabled(true);

                        tracker.herocapes$setCustomCapeTexture(new ResourceLocation(HeroCapes.MODID,
                                "textures/cape/iron_chestplate_cape.png"));
                        // Prevent the player from using cape commands, don't use herocapes$setCapeTexture!!!

                        tracker.herocapes$setCapeHeight(25);
                        tracker.herocapes$setGravity(-0.025f);
                        //...
                    }
                    else {
                        tracker.herocapes$setCustomCapeTexture(null); // Allow the player to use cape commands or don't...
                        tracker.herocapes$setCapeHeight(21);
                        tracker.herocapes$setGravity(-0.035f);
                        //...
                    }
                }
            }
        }
    }