DEPRECATE: remove NTLM_WB in June 2024

Ref: https://curl.se/mail/lib-2023-12/0010.html

Closes #12451

docs/DEPRECATE.md

@@ -6,6 +6,17 @@ email the
 as soon as possible and explain to us why this is a problem for you and
 how your use case cannot be satisfied properly using a workaround.
 
+## NTLM_WB auth
+
+This NTLM authentication method is powered by a separate tool,
+`ntlm_auth`. Barely anyone uses this method. It was always a quirky
+implementation (including fork + exec), it has limited portability and we
+don't test it in the test suite and CI.
+
+We keep the native NTLM implementation.
+
+curl will remove the support for NTLM_WB auth in June 2024.
+
 ## space-separated `NOPROXY` patterns
 
 When specifying patterns/domain names for curl that should *not* go through a