Wordpress - WordPress adding scaled images that don't exist (1536x1536 and 2048x2048)

I found the culprit!

WordPress 5.3 introduced additional image sizes which can be found via /wp-includes/media.php.

Updating my function, like so, removed the extra sizes:

function remove_default_image_sizes( $sizes) {
    unset( $sizes['large']); // Added to remove 1024
    unset( $sizes['thumbnail']);
    unset( $sizes['medium']);
    unset( $sizes['medium_large']);
    unset( $sizes['1536x1536']);
    unset( $sizes['2048x2048']);
    return $sizes;
}
add_filter('intermediate_image_sizes_advanced', 'remove_default_image_sizes');

You could also remove those image sizes completely using remove_image_size (see: https://developer.wordpress.org/reference/functions/remove_image_size/)

Example (to be placed in your functions.php file):

remove_image_size('1536x1536');
remove_image_size('2048x2048');

This function, however, won't work for default WP image sizes (e.g. 'thumbnail', 'medium', 'large', etc.). There's a work-around though. Simply set the sizes to 0:

update_option( 'thumbnail_size_h', 0 );
update_option( 'thumbnail_size_w', 0 );
update_option( 'medium_size_h', 0 );
update_option( 'medium_size_w', 0 );
update_option( 'medium_large_size_w', 0 );
update_option( 'medium_large_size_h', 0 );
update_option( 'large_size_h', 0 );
update_option( 'large_size_w', 0 );