How to exclude an image size from the WordPress srcset

WordPress allows you to hook into wp_calculate_image_srcset

See Core file on Trac

This filter gives you 5 arguments:

• $sources One or more arrays of source data to include in the ‘srcset’
• $size_array Array of width and height values in pixels (in that order).
• $image_src The ‘src’ of the image.
• $image_meta The image meta data as returned by ‘wp_get_attachment_metadata()
• $attachment_id the post_id from the image

Here is a sample code you can use within your themes function.php or plugin files to get started:

add_filter( 'wp_calculate_image_srcset', 'my_custom_image_srcset', 10, 5);

function my_custom_image_srcset($sources, $size_array, $image_src, $image_meta, $attachment_id) {
    // The following code is an adaption from wp-includes/media.php:1061-1180
    $image_sizes = $image_meta['sizes'];

    // Get the width and height of the image.
    $image_width = (int) $size_array[0];
    $image_height = (int) $size_array[1];

    $image_basename = wp_basename( $image_meta['file'] );

    /*
     * WordPress flattens animated GIFs into one frame when generating intermediate sizes.
     * To avoid hiding animation in user content, if src is a full size GIF, a srcset attribute is not generated.
     * If src is an intermediate size GIF, the full size is excluded from srcset to keep a flattened GIF from becoming animated.
     */
    if ( ! isset( $image_sizes['thumbnail']['mime-type'] ) || 'image/gif' !== $image_sizes['thumbnail']['mime-type'] ) {
        $image_sizes[] = array(
            'width'  => $image_meta['width'],
            'height' => $image_meta['height'],
            'file'   => $image_basename,
        );
    } elseif ( strpos( $image_src, $image_meta['file'] ) ) {
        return false;
    }

    // Retrieve the uploads sub-directory from the full size image.
    $dirname = _wp_get_attachment_relative_path( $image_meta['file'] );

    if ( $dirname ) {
        $dirname = trailingslashit( $dirname );
    }

    $upload_dir = wp_get_upload_dir();
    $image_baseurl = trailingslashit( $upload_dir['baseurl'] ) . $dirname;

    /*
     * If currently on HTTPS, prefer HTTPS URLs when we know they're supported by the domain
     * (which is to say, when they share the domain name of the current request).
     */
    if ( is_ssl() && 'https' !== substr( $image_baseurl, 0, 5 ) && parse_url( $image_baseurl, PHP_URL_HOST ) === $_SERVER['HTTP_HOST'] ) {
        $image_baseurl = set_url_scheme( $image_baseurl, 'https' );
    }

    /*
     * Images that have been edited in WordPress after being uploaded will
     * contain a unique hash. Look for that hash and use it later to filter
     * out images that are leftovers from previous versions.
     */
    $image_edited = preg_match( '/-e[0-9]{13}/', wp_basename( $image_src ), $image_edit_hash );

    /**
     * Filters the maximum image width to be included in a 'srcset' attribute.
     *
     * @since 4.4.0
     *
     * @param int   $max_width  The maximum image width to be included in the 'srcset'. Default '1600'.
     * @param array $size_array Array of width and height values in pixels (in that order).
     */
    $max_srcset_image_width = apply_filters( 'max_srcset_image_width', 1600, $size_array );

    // Array to hold URL candidates.
    $sources = array();

    /**
     * To make sure the ID matches our image src, we will check to see if any sizes in our attachment
     * meta match our $image_src. If no matches are found we don't return a srcset to avoid serving
     * an incorrect image. See #35045.
     */
    $src_matched = false;

    /*
     * Loop through available images. Only use images that are resized
     * versions of the same edit.
     */
    foreach ( $image_sizes as $identifier => $image ) {
        // Continue if identifier is unwanted
        if ($identifier === 'unwanted-identifier') {
            continue;
        }

        $is_src = false;

        // Check if image meta isn't corrupted.
        if ( ! is_array( $image ) ) {
            continue;
        }

        // If the file name is part of the `src`, we've confirmed a match.
        if ( ! $src_matched && false !== strpos( $image_src, $dirname . $image['file'] ) ) {
            $src_matched = $is_src = true;
        }

        // Filter out images that are from previous edits.
        if ( $image_edited && ! strpos( $image['file'], $image_edit_hash[0] ) ) {
            continue;
        }

        /*
         * Filters out images that are wider than '$max_srcset_image_width' unless
         * that file is in the 'src' attribute.
         */
        if ( $max_srcset_image_width && $image['width'] > $max_srcset_image_width && ! $is_src ) {
            continue;
        }

        // If the image dimensions are within 1px of the expected size, use it.
        if ( wp_image_matches_ratio( $image_width, $image_height, $image['width'], $image['height'] ) ) {
            // Add the URL, descriptor, and value to the sources array to be returned.
            $source = array(
                'url'        => $image_baseurl . $image['file'],
                'descriptor' => 'w',
                'value'      => $image['width'],
            );

            // The 'src' image has to be the first in the 'srcset', because of a bug in iOS8. See #35030.
            if ( $is_src ) {
                $sources = array( $image['width'] => $source ) + $sources;
            } else {
                $sources[ $image['width'] ] = $source;
            }
        }
    }
    return $sources;
}