tcp_input() and in6_ifawithscope() belong to input and output path
respectively. Not used in ramdisk.

Index: sys/netinet/tcp_input.c
===================================================================
RCS file: /cvs/src/sys/netinet/tcp_input.c,v
retrieving revision 1.456
diff -u -p -r1.456 tcp_input.c
--- sys/netinet/tcp_input.c	8 Jul 2025 00:47:41 -0000	1.456
+++ sys/netinet/tcp_input.c	24 Jul 2025 20:33:13 -0000
@@ -847,7 +847,8 @@ findpcb:
 				 * are not exactly the same.  I believe 5.5.4
 				 * is the best one, so we follow 5.5.4.
 				 */
-				if (ip6 && !ip6_use_deprecated) {
+				if (ip6 &&
+				    !atomic_load_int(&ip6_use_deprecated)) {
 					struct in6_ifaddr *ia6;
 					struct ifnet *ifp =
 					    if_get(m->m_pkthdr.ph_ifidx);
Index: sys/netinet6/in6.c
===================================================================
RCS file: /cvs/src/sys/netinet6/in6.c,v
retrieving revision 1.271
diff -u -p -r1.271 in6.c
--- sys/netinet6/in6.c	8 Jul 2025 00:47:41 -0000	1.271
+++ sys/netinet6/in6.c	24 Jul 2025 20:33:13 -0000
@@ -1427,7 +1427,7 @@ in6_ifawithscope(struct ifnet *oifp, con
 				 * Ignore any deprecated addresses if
 				 * specified by configuration.
 				 */
-				if (!ip6_use_deprecated)
+				if (!atomic_load_int(&ip6_use_deprecated))
 					continue;
 
 				/*
Index: sys/netinet6/in6_proto.c
===================================================================
RCS file: /cvs/src/sys/netinet6/in6_proto.c,v
retrieving revision 1.142
diff -u -p -r1.142 in6_proto.c
--- sys/netinet6/in6_proto.c	24 Jul 2025 19:49:08 -0000	1.142
+++ sys/netinet6/in6_proto.c	24 Jul 2025 20:33:13 -0000
@@ -361,7 +361,7 @@ int	ip6_hdrnestlimit = 10;	/* [a] approp
 int	ip6_dad_count = 1;	/* [a] DupAddrDetectionTransmits */
 int	ip6_dad_pending;	/* number of currently running DADs */
 int	ip6_auto_flowlabel = 1;	/* [a] */
-int	ip6_use_deprecated = 1;	/* allow deprecated addr (RFC2462 5.5.4) */
+int	ip6_use_deprecated = 1;	/* [a] allow deprecated addr (RFC2462 5.5.4) */
 int	ip6_mcast_pmtu = 0;	/* enable pMTU discovery for multicast? */
 int	ip6_neighborgcthresh = 2048; /* Threshold # of NDP entries for GC */
 int	ip6_maxdynroutes = 4096; /* Max # of routes created via redirect */
Index: sys/netinet6/ip6_input.c
===================================================================
RCS file: /cvs/src/sys/netinet6/ip6_input.c,v
retrieving revision 1.286
diff -u -p -r1.286 ip6_input.c
--- sys/netinet6/ip6_input.c	24 Jul 2025 19:49:08 -0000	1.286
+++ sys/netinet6/ip6_input.c	24 Jul 2025 20:33:13 -0000
@@ -1453,10 +1453,10 @@ const struct sysctl_bounded_args ipv6ctl
 	{ IPV6CTL_DAD_COUNT, &ip6_dad_count, 0, 10 },
 	{ IPV6CTL_AUTO_FLOWLABEL, &ip6_auto_flowlabel, 0, 1 },
 	{ IPV6CTL_DEFMCASTHLIM, &ip6_defmcasthlim, 0, 255 },
+	{ IPV6CTL_USE_DEPRECATED, &ip6_use_deprecated, 0, 1 },
 };
 
 const struct sysctl_bounded_args ipv6ctl_vars[] = {
-	{ IPV6CTL_USE_DEPRECATED, &ip6_use_deprecated, 0, 1 },
 	{ IPV6CTL_MAXFRAGS, &ip6_maxfrags, 0, 1000 },
 	{ IPV6CTL_MFORWARDING, &ip6_mforwarding, 0, 1 },
 	{ IPV6CTL_MCAST_PMTU, &ip6_mcast_pmtu, 0, 1 },
@@ -1576,6 +1576,7 @@ ip6_sysctl(int *name, u_int namelen, voi
 	case IPV6CTL_DAD_COUNT:
 	case IPV6CTL_AUTO_FLOWLABEL:
 	case IPV6CTL_DEFMCASTHLIM:
+	case IPV6CTL_USE_DEPRECATED:
 		return (sysctl_bounded_arr(
 		    ipv6ctl_vars_unlocked, nitems(ipv6ctl_vars_unlocked),
 		    name, namelen, oldp, oldlenp, newp, newlen));