This script removes each ytd-rich-section-renderer element that has an element with the id "dismissible". These sections usually show promoted content.

yt-remove-rich-section.meta.js

// ==UserScript==
// @name         YouTube - Remove rich section with dismissible elements
// @namespace    https://example.org/userscripts
// @version      1.1.0
// @author       Léa Gris
// @license      0BSD
// @updateURL    https://gist.github.com/leagris/d4d52ab5b17cd762fcf911793e0f8dc1/raw/yt-remove-rich-section.meta.js
// @downloadURL  https://gist.github.com/leagris/d4d52ab5b17cd762fcf911793e0f8dc1/raw/yt-remove-rich-section.user.js
// @description  This script removes each ytd-rich-section-renderer element that has an element with the id "dismissible". These sections usually show promoted content.
// @match        https://www.youtube.com/*
// @match        https://m.youtube.com/*
// @run-at       document-start
// @grant        none
// ==/UserScript==

yt-remove-rich-section.user.js

// ==UserScript==
// @name         YouTube - Remove rich section with dismissible elements
// @namespace    https://example.org/userscripts
// @version      1.1.0
// @author       Léa Gris
// @license      0BSD
// @updateURL    https://gist.github.com/leagris/d4d52ab5b17cd762fcf911793e0f8dc1/raw/yt-remove-rich-section.meta.js
// @downloadURL  https://gist.github.com/leagris/d4d52ab5b17cd762fcf911793e0f8dc1/raw/yt-remove-rich-section.user.js
// @description  This script removes each ytd-rich-section-renderer element that has an element with the id "dismissible". These sections usually show promoted content.
// @match        https://www.youtube.com/*
// @match        https://m.youtube.com/*
// @run-at       document-start
// @grant        none
// ==/UserScript==
(function () {
    'use strict';
    const RICH_SECTION = 'ytd-rich-section-renderer';
    function hideDismissibleRichSections() {
        if (!window.CSS?.supports?.('selector(:has(*))')) {
            return;
        }
        const sheet = new CSSStyleSheet();
        const index = sheet.insertRule(`${RICH_SECTION}:has([id="dismissible"]) {}`);
        sheet.cssRules[index].style.setProperty('display', 'none', 'important');
        document.adoptedStyleSheets = [...document.adoptedStyleSheets, sheet];
    }
    function removeSectionIfDismissible(section) {
        if (section.querySelector('#dismissible')) {
            section.remove();
        }
    }
    function onChildListMutationRemoveDismissibleSections(mutations) {
        for (const mutation of mutations) {
            // YouTube can put the dismissible element in the page after its section.
            const parentSection = mutation.target.closest?.(RICH_SECTION);
            if (parentSection) {
                removeSectionIfDismissible(parentSection);
            }
            for (const node of mutation.addedNodes) {
                if (node.nodeType !== Node.ELEMENT_NODE) {
                    continue;
                }
                if (node.matches(RICH_SECTION)) {
                    removeSectionIfDismissible(node);
                }
                for (const section of node.querySelectorAll(RICH_SECTION)) {
                    removeSectionIfDismissible(section);
                }
            }
        }
    }
    hideDismissibleRichSections();
    new MutationObserver(onChildListMutationRemoveDismissibleSections)
        .observe(document.documentElement, { childList: true, subtree: true });
})();