When I use:

define('WP_DEBUG', 1);

In my wp-config.php, it works fine, but I am hacking an old theme and I would like to suppress deprecated notices.

My understanding is that adding this:

error_reporting( E_ERROR | E_NOTICE | E_PARSE )

Should do the trick. I have added it to wp-config.php and to header.php in my theme. Unfortunately, it has no effect. Is this something set at server level? Also the following makes no difference as well:

ini_set('display_errors', 1);