remove-youtube-number-hotkeys.user.js

// ==UserScript==
// @name        Remove YouTube Number Hotkeys
// @match       https://www.youtube.com/watch
// @run-at      document-end
// ==/UserScript==

document.addEventListener('keydown', (event) => {
  if (event.target.id === 'movie_player' &&
      ["1", "2", "3", "4", "5", "6", "7", "8", "9"].includes(event.key)) {
    event.stopPropagation();
  }
}, true);