A small snippet to notify users if they accidentally installed Secure Custom Fields instead of Advanced Custom Fields.

check_scf_upgrade.php

<?php

if ( ! function_exists( 'check_scf_upgrade' ) ) :

	function check_scf_upgrade() {
		add_action( 'admin_notices', function(){
			if(!get_user_meta(get_current_user_id(), 'acf_upgrade_notice_dismissed', true) && class_exists( 'ACF_Admin' ) && !method_exists('ACF_Admin', 'setup_help_tab')) {
				?>
				<div class="notice notice-error acf-upgrade-notice is-dismissible">
					<p><strong>Advanced Custom Fields is no longer available through WordPress.org.</strong> Please update to the latest version following these instructions:</p>
					<p><a href="https://www.advancedcustomfields.com/blog/installing-and-upgrading-to-the-latest-version-of-acf/" target="_blank">https://www.advancedcustomfields.com/blog/installing-and-upgrading-to-the-latest-version-of-acf/</a></p>
				</div>
				<?php
			}
		});	

		add_action('admin_footer', function(){
			$nonce = wp_create_nonce('acf_upgrade_notice_nonce');
			?>
				<script>
					jQuery(document).ready(function($) {
						$('.acf-upgrade-notice').on('click', '.notice-dismiss', function(){
							$.ajax({
								url: ajaxurl,
								type: 'POST',
								data: {
									action: 'dismiss_acf_upgrade_notice',
									nonce: '<?php echo esc_attr($nonce); ?>'
								}
							});
						});
					});
				</script>
			<?php
		});

		add_action('wp_ajax_dismiss_acf_upgrade_notice', function() {
			check_ajax_referer('acf_upgrade_notice_nonce', 'nonce');
			update_user_meta(get_current_user_id(), 'acf_upgrade_notice_dismissed', true);
			wp_send_json_success();
		});	
	};

	check_scf_upgrade();
endif;