Disable srcset generation

functions.php

<?php
// Add to your functions.php

// Disable SRCSET
add_filter('wp_get_attachment_image_attributes', function($attr) {
    if (isset($attr['sizes'])) {
        unset($attr['sizes']);
    }

    if (isset($attr['srcset'])) {
        unset($attr['srcset']);
    }

    return $attr;
}, PHP_INT_MAX);

// Override the calculated image sources
add_filter('wp_calculate_image_srcset', '__return_empty_array', PHP_INT_MAX);