Deprecated t.isFunction Call on D11.1

What’s blowing up?
Drupal 11 ships with jQuery 4.0.0-beta. One of the breaking changes in jQuery 4 is that the long-deprecated helper jQuery.isFunction() (and the alias $.isFunction) has been removed. The minified Waypoints 4.0.1 library that your theme/module loads still calls that helper. In the compressed file the call appears as t.isFunction, where t is the local reference to jQuery. Because the method no longer exists, the browser throws:
Uncaught TypeError: t.isFunction is not a function

right where Waypoints tries to register a waypoint.

Fix options:

1. Patch Waypoints (recommended)
Replace every occurence of $.isFunction(…) with typeof … === 'function'

2. Add a one-line polyfill Before Waypoints loads, inject js
js if (typeof jQuery.isFunction !== 'function') { jQuery.isFunction = obj => typeof obj === 'function'; }

3. Use jQuery Migrate 4 plugin Load the official migrate plugin that re-adds removed helpers.

Quick Drupal-specific patch
Create my_module/js/jquery4-polyfills.js:

/**
 * Tiny polyfills for libraries that still expect jQuery < 4.
 */
(( $, Drupal, drupalSettings ) => {
  if (typeof $.isFunction !== 'function') {
    $.isFunction = obj => typeof obj === 'function';
  }
})(jQuery, Drupal, drupalSettings);

Then add this library to your theme/modules’ libraries.yml before the Waypoints library to guarantee it is evaluated first.

Long-term
Track the “Deprecated t.isFunction call on D11” issue in the jQuery Waypoints queue; once a new release lands, remove your patch/polyfill.
Drupal

Audit any other contributed or custom scripts for calls to $.isFunction, $.isNumeric, $.trim, jQuery.type, etc.—all of them vanished in jQuery 4. Drupal’s upgrade status report will flag many of these automatically.
Drupal

Once Waypoints (and any similar libraries) are patched, Drupal 11 + jQuery 4 will run without the “isFunction” crash.