let ticks = 0;
//....

mod.on("update", () => {
    ticks++;

    if (ticks === 10) { // put it as your opinion
        mc.gameSettings.keyBindSneak.pressed = true;
    } else {
        mc.gameSettings.keyBindSneak.pressed = false;
        ticks = 0;
    }
}

const script = registerScript({
    name: "AntiVulcanLimit",
    version: "1.0.0",
    authors: ["IgniteTNT"]
});

var sneakCooldown = 200; // Cooldown in milliseconds 

script.registerModule({
    name: "VulcanLimit",
    category: "Misc", // Movement, Combat, Renderr, ...
    description: "Bypass Vulcan's Limit Check When Scaffolding"
}, (mod) => {
    mod.on("enable", () => {
        Client.displayChatMessage("§aEnabled AntiVulcanLimit :)");
        mc.gameSettings.keyBindSneak.pressed = true;
        setTimeout(function() {
            mc.gameSettings.keyBindSneak.pressed = false;
        }, sneakCooldown);
    });

    mod.on("disable", () => {
        Client.displayChatMessage("§aDisabled AntiVulcanLimit :(");
    });
    
    mod.on("ground", function(event) {
        mc.gameSettings.keyBindSneak.pressed = true;
        setTimeout(function() {
            mc.gameSettings.keyBindSneak.pressed = false;
        }, sneakCooldown);
    });
});

In this corrected version, I replaced this.onGround with mod.on("ground", ...), which correctly registers the event handler function.

NEXTGEN: https://liquidbounce.net/docs/ScriptAPI (nextgen)