Tiny script to unfollow all people you're following. Just go to https://twitter.com/<youruser>/following, open the console and run the code (testing in responsive mode).

tw-unfollower.js

function unfollow() {
    let count = 0
    const buttons = document.querySelectorAll('[data-testid*="-unfollow"]')
    
    if (!buttons.length) {
        console.log(`${count} People unfollowed`)
        return
    }
    
    const waitFor = (ms) => new Promise(resolve => setTimeout(resolve, ms))
    
    const acceptModals = async (index = 0) => {
        if (index === buttons.length - 1) {
            return
        }
        const confirmBtn = document.querySelector('[data-testid*="confirmationSheetConfirm"]')
        confirmBtn.click()
        count += 1
        await waitFor(150) // you can play with this timing if you don't have the desired results.
        return acceptModals(index + 1)
    }
    
    for (const button of buttons) {
        button.click()
    }
    
    acceptModals().then(unfollow, console.err)
}

unfollow()