Go to https://www.youtube.com/feed/channels, scroll all the way to the bottom then run this script to clear all of your subscriptions.

Clear all youtube subscriptions.js

const unSubscribe = () => {
    Array.from(document.querySelectorAll("yt-formatted-string"))
        .find(yfs => yfs.textContent === "Subscribed")
        .click();

    setTimeout(() => {
        Array.from(document.querySelectorAll("yt-formatted-string"))
            .filter(yfs => yfs.textContent === "Unsubscribe")
            .forEach(yfs => yfs.click());
    }, 400);
};


const amountOfSubscriptions = Array.from(document.querySelectorAll("yt-formatted-string"))
    .filter(yfs => yfs.textContent === "Subscribed")
    .length;

let currentSubscriptionNumber = 1;

const unSubScribeAll = () => {
    setTimeout(() => {
        unSubscribe();
        console.log(`${amountOfSubscriptions - currentSubscriptionNumber} remaining.`);
        currentSubscriptionNumber++;
        unSubScribeAll();
    }, 1000);
};


console.log(`Removing ${amountOfSubscriptions} subscriptions.`);

unSubScribeAll();