Hmm, the original deprecation commit message might shed a light why this doesn't cause any problems:
"$NS is only used in lib/exe/{ajax,mediamanager}.php when no $ID context
is present. The two functions which use $NS in inc/template.php are only
called through those both endpoints, not through doku.php. In doku.php,
$ID is the only correct value, $NS is not kept synchronous with $ID."
See
https://github.com/splitbrain/dokuwiki/commit/bced4159c8c5f1b4da2e98b2242c2ec1c09ec74e