How to remove the “+Add Category” button from the Category metabox?

I installed the ‘User Role Editor’ plugin and removed the ‘manage_categories’ capability for the ‘Editor’ which worked. But I would like to remove it for ALL users including admin, superadmin.

If removing the ‘manage_categories’ capability from the editor role provides the functionality you want, then you can remove the ‘manage_categories’ capability from all user roles fairly easily either by using an existing plugin, or writing a new plugin that upon activation removes the capability from all the user roles. Here’s the code that would be necessary to remove the capability from each role (actually, it explicitly sets the capability to false).

register_activation_hook( __FILE__, 'wpse_259647_remove_manage_categories_cap' );
function wpse_259647_remove_manage_categories_cap() {
  $roles = wp_roles();
  foreach( $roles->role_names as $slug => $name ) {
    $role = get_role( $slug );
    $role->add_cap( 'manage_categories', false );
  }
}

This won’t remove the capability from “super admins” though since “super admins” technically aren’t a role or a capability.

If you want to explicitly revoke the manage_categories capability from all users, you could do that too.

register_activation_hook( __FILE__, 'wpse_259647_remove_manage_categories_cap' );
function wpse_259647_remove_manage_categories_cap() {
  $users = get_users();
  foreach( $users as $user ) {
    $user->add_cap( 'manage_categories', false );
  }
}

This may still not remove the capability from “super admins” though, because WordPress treats them differently than other users.