Disable WooCommerce payment method using warning/alert message instead of removing. Show the payment method, just disable it with a warning/alert message.

README.md

Disable WooCommerce Payment Method Using Message

Functionality

• Inserts a custom message if the user selects that payment method
• Actually disables WooCommerce from using a specific payment method (no user HTML workarounds)
• Prevents the Place order button from being clicked for that payment method, allows other methods to proceed
• Skips over the payment method automatically when the checkout page loads

Install

1. Add the PHP code into your theme's functions.php file or by using a plugin like Theme Customisations.
2. Change the DISABLE_GATEWAY_ID to the ID of whichever payment gateway you want to disable.
3. Change the DISABLE_MESSAGE to whatever message you want to appear on the disabled payment gateway.
4. Optionally use the CSS in style.css to adjust the margins of the alert box

Disable-WooCommerce-Payment-Method-Message.php

/****************************************************************************************
 * START - Disable a payment gateway and show a custom message in WooCommerce
 ****************************************************************************************/

/**
 * Define constants at the top for easy configuration.
 */
define('DISABLE_GATEWAY_ID', 'plisio');
define('DISABLE_MESSAGE', '<div class="woocommerce-error">Cryptocurrency payments are temporarily unavailable while we improve the payment process.</div>');

add_filter('woocommerce_available_payment_gateways', 'disable_payment_gateway', 10, 1);

function disable_payment_gateway($available_gateways) {
    if (isset($available_gateways[DISABLE_GATEWAY_ID])) {
        // Disable the payment method but still display it
        $available_gateways[DISABLE_GATEWAY_ID]->enabled = false;

        // Add custom message to the description
        $available_gateways[DISABLE_GATEWAY_ID]->description = DISABLE_MESSAGE . $available_gateways[DISABLE_GATEWAY_ID]->description;
    }
    return $available_gateways;
}

add_action('wp_footer', 'disable_checkout_button_for_payment_method');

function disable_checkout_button_for_payment_method() {
    if (is_checkout()) {
    ?>
        <script type="text/javascript">
            jQuery(document).ready(function($) {
                // Continuously check for changes in payment method selection
                $('form.checkout').on('change', 'input[name="payment_method"]', function() {
                    // Check if the disabled payment method is selected
                    if ($('input[name="payment_method"]:checked').val() === '<?php echo DISABLE_GATEWAY_ID; ?>') {
                        // Disable the place order button
                        $('#place_order').prop('disabled', true).css('opacity', '0.5');
                    } else {
                        // Enable the place order button if other methods are selected
                        $('#place_order').prop('disabled', false).css('opacity', '1');
                    }
                });

                // Trigger the change event on page load in case the method is already selected
                $('input[name="payment_method"]:checked').trigger('change');
            });
        </script>
<?php
    }
}

/****************************************************************************************
 * END - Disable a payment gateway and show a custom message in WooCommerce
 ****************************************************************************************/

style.css

.woocommerce-message, .woocommerce-info, .woocommerce-error {
    margin-bottom: 1em;
}