How to Redirect all 404 pages to home?

You can handle all broken 404 pages to the home page or another page using the “Customize” feature.
Go to the section Customize > Themes > Create New Theme > name your theme ex 404 Redirect and set the Base Theme to Default.
Then create the new theme > Edit CSS/HTML. In the HTML section, create a JavaScript snippet that will handle the redirection.
Past this code.

<script type="text/discourse-plugin">
  api.onPageChange((url, title) => {
    // Redirect to the home page for all 404 pages
    if (url.includes('/404')) {
      window.location.href = '/';

Please adjust your page name according to your need. Read this redirect old url to new discourse url