This routine does not seem to be used anymore.

Index: sys/kern/uipc_mbuf.c
===================================================================
RCS file: /OpenBSD/src/sys/kern/uipc_mbuf.c,v
diff -u -p -u -p -r1.307 uipc_mbuf.c
--- sys/kern/uipc_mbuf.c	3 Jul 2026 11:51:57 -0000	1.307
+++ sys/kern/uipc_mbuf.c	20 Sep 2026 16:47:44 -0000
@@ -1731,18 +1731,6 @@ ml_dequeue(struct mbuf_list *ml)
 	return (m);
 }
 
-struct mbuf *
-ml_dechain(struct mbuf_list *ml)
-{
-	struct mbuf *m0;
-
-	m0 = ml->ml_head;
-
-	ml_init(ml);
-
-	return (m0);
-}
-
 unsigned int
 ml_purge(struct mbuf_list *ml)
 {
Index: sys/sys/mbuf.h
===================================================================
RCS file: /OpenBSD/src/sys/sys/mbuf.h,v
diff -u -p -u -p -r1.271 mbuf.h
--- sys/sys/mbuf.h	3 Jul 2026 11:51:57 -0000	1.271
+++ sys/sys/mbuf.h	20 Sep 2026 16:47:44 -0000
@@ -535,7 +535,6 @@ void			ml_init(struct mbuf_list *);
 void			ml_enqueue(struct mbuf_list *, struct mbuf *);
 struct mbuf *		ml_dequeue(struct mbuf_list *);
 void			ml_enlist(struct mbuf_list *, struct mbuf_list *);
-struct mbuf *		ml_dechain(struct mbuf_list *);
 unsigned int		ml_purge(struct mbuf_list *);
 unsigned int		ml_hdatalen(struct mbuf_list *);