Per https://www.iana.org/assignments/iana-ipv6-special-registry/

3fff::/20 has been set aside as an additional documentation prefix

https://datatracker.ietf.org/doc/html/draft-ietf-v6ops-rfc3849-update

Index: bgpd.conf
===================================================================
RCS file: /cvs/src/etc/examples/bgpd.conf,v
diff -u -p -r1.22 bgpd.conf
--- bgpd.conf	27 Sep 2023 10:49:21 -0000	1.22
+++ bgpd.conf	24 Jul 2024 18:49:13 -0000
@@ -39,6 +39,7 @@ prefix-set bogons {
 	2001:db8::/32 or-longer		# docu range [RFC3849]
 	2002::/16 or-longer		# 6to4 anycast relay [RFC7526]
 	3ffe::/16 or-longer		# old 6bone
+	3fff::/20 or-longer		# docu range [draft-ietf-v6ops-rfc3849-update]
 	fc00::/7 or-longer		# unique local unicast
 	fe80::/10 or-longer		# link local unicast
 	fec0::/10 or-longer		# old site local unicast