- Delayed-ack is now a compile-time option, compiled out by default.

This feature is simply too experimental for right now, and causes
  some problems to some failover installations.  We will revisit this
  in future releases.  [ISC-Bugs #18832]

server/stables.c

@@ -239,8 +239,11 @@ static struct option server_options[] = {
 	{ "dhcpv6-pid-file-name", "t",		&server_universe,  55, 1 },
 	{ "limit-addrs-per-ia", "L",		&server_universe,  56, 1 },
 	{ "limit-prefs-per-ia", "L",		&server_universe,  57, 1 },
+/* Assert a configuration parsing error if delayed-ack isn't compiled in. */
+#if defined(DELAYED_ACK)
 	{ "delayed-ack", "S",			&server_universe,  58, 1 },
 	{ "max-ack-delay", "L",			&server_universe,  59, 1 },
+#endif
 	{ NULL, NULL, NULL, 0, 0 }
 };