Can find implementation of Slang in rakudo

https://github.com/rakudo/rakudo/blob/8ff76b5962ed34caa7b66b3f95644410fdb9b17e/src/core/Slang.pm

Author: tisonkun

doc/Language/variables.pod6

@@ -399,8 +399,6 @@ C<Associative> roles.
 =head2 The C<~> Twigil
 X<|$~>
 
-Note: Slangs are L<NYI|/language/glossary#NYI> in Rakudo.
-
 The C<~> twigil is for referring to sublanguages (called slangs). The
 following are useful: