// Removes the decoding attribute from images added inside post content.

add_filter( 'wp_img_tag_add_decoding_attr', '__return_false' );

​

// Remove the decoding attribute from featured images and the Post Image block.

add_filter( 'wp_get_attachment_image_attributes', function( $attributes ) {

    unset( $attributes['decoding'] );

    return $attributes;

} );