example-basic-user-avatar-alt-tag.php

<?php
/**
 * Example of how to set an alt tag for Basic User Avatars when the
 * alt tag is not set.
 *
 * You can add this recipe to your site by creating a custom plugin
 * or using the Code Snippets plugin available for free in the WordPress repository.
 * Read this companion article for step-by-step directions on either method.
 * https://www.paidmembershipspro.com/create-a-plugin-for-pmpro-customizations/
 */

function my_basic_user_avatar_alt_tag( $arguments ) {
	if ( empty( $arguments['alt'] ) ) {
		$arguments['alt'] = 'User avatar';
	}

	return $arguments;
}
add_filter( 'basic_user_avatar_data', 'my_basic_user_avatar_alt_tag', 10, 1 );