Works with latest version of WP?

Hello,

I’ve been having the same problem on WP 4.4.2, thus I got the plugin finally working and I think the main problem is with the admin_footer hook used.
Anyway change the username_check.php file in the plugin directory to the following:

add_action('wp_head', 'usercheck');

function usercheck() {
    ?>
    <script type="text/javascript" >
        var xhr = 0;
        jQuery(document).ready(function ($) {
            $('#user_login, input[name="user_login"]')
                    .after('<span class="uc_disp" id="uc_ajax_loader">'
                            + '<img src="<?php echo plugin_dir_url(__FILE__) . 'ajax-loader.gif'; ?>" width=16 height=16 title="Loading..."/></span>')
                    .after('<span class="uc_disp" id="uc_valid">'
                            + '<img src="<?php echo plugin_dir_url(__FILE__) . 'valid.png'; ?>" width=16 height=16 title="Valid"/></span>')
                    .after('<span class="uc_disp" id="uc_invalid">'
                            + '<img src="<?php echo plugin_dir_url(__FILE__) . 'invalid.png'; ?>" width=16 height=16 title="Invalid"/></span>')
            $('.uc_disp').hide();
            $('#user_login').bind('keyup change', function () {
                var data = {
                    action: 'check_username',
                    username: $('#user_login').val(),
                };
                $('.uc_disp').hide();
                $('#uc_ajax_loader').show();
                if (xhr)
                    xhr.abort();
                xhr = $.ajax({
                    url: ajaxurl,
                    type: 'get',
                    async: true,
                    data: data,
                    success: function (response) {
                        $('.uc_disp').hide();
                        if (response == 0) {
                            $('#uc_valid').show();
                            $('input[type="submit"]').removeClass("disabled");
                            $('input[type="submit"]').prop("disabled", false);
                        } else {
                            $('#uc_invalid').show();
                            $('input[type="submit"]').addClass("disabled");
                            $('input[type="submit"]').prop("disabled", true);
                            $('input[type="submit"]').attr("title", "the username entered is not available");
                        }
                        ;
                    },
                });
            });

        });
    </script>
    <?php
}

//echo 0 si n'existe pas
add_action('wp_ajax_check_username', function() {
    if (username_exists($_GET['username']) == null)
        echo 0;
    else
        echo 1;
    die();
});

This will also work on Woocommerce username field in the registration form (I double checked).

In addition I added a disabled attribute, a disabled class, an info title to the register button whenever an unavailable username is chosen.

I hope that helps and best of luck,
Cheers!