What "readiness test" are you using to determine if it's OK?
In most cases, users pass out any ICMP which should take care of that. pf is usually smart enough to let through ICMP errors related to connections with open states.
We could consider a patch like the following to strictly allow what RFC 4890 claims should be allowed, though I'm not convinced it's necessary given the way most people intend a firewall to operate.
diff --git a/src/etc/inc/filter.inc b/src/etc/inc/filter.inc
index 6890c98..08625d4 100644
--- a/src/etc/inc/filter.inc
+++ b/src/etc/inc/filter.inc
@@ -3118,13 +3118,20 @@ EOD;
# See man icmp6(4)
# 1 unreach Destination unreachable
# 2 toobig Packet too big
+# 3 timex Time exceeded
+# 4 paramprob Parameter problem
# 128 echoreq Echo service request
# 129 echorep Echo service reply
# 133 routersol Router solicitation
# 134 routeradv Router advertisement
# 135 neighbrsol Neighbor solicitation
# 136 neighbradv Neighbor advertisement
-pass {$log['pass']} quick inet6 proto ipv6-icmp from any to any icmp6-type {1,2,135,136} tracker {$increment_tracker($tracker)} keep state
+pass {$log['pass']} quick inet6 proto ipv6-icmp from any to any icmp6-type {1,2,128,129} tracker {$increment_tracker($tracker)} keep state
+pass {$log['pass']} quick inet6 proto ipv6-icmp from any to any icmp6-type 3 code 0 tracker {$increment_tracker($tracker)} keep state
+pass {$log['pass']} quick inet6 proto ipv6-icmp from any to any icmp6-type 3 code 1 tracker {$increment_tracker($tracker)} keep state
+pass {$log['pass']} quick inet6 proto ipv6-icmp from any to any icmp6-type 4 code 0 tracker {$increment_tracker($tracker)} keep state
+pass {$log['pass']} quick inet6 proto ipv6-icmp from any to any icmp6-type 4 code 1 tracker {$increment_tracker($tracker)} keep state
+pass {$log['pass']} quick inet6 proto ipv6-icmp from any to any icmp6-type 4 code 2 tracker {$increment_tracker($tracker)} keep state
# Allow only bare essential icmpv6 packets (NS, NA, and RA, echoreq, echorep)
pass out {$log['pass']} quick inet6 proto ipv6-icmp from fe80::/10 to fe80::/10 icmp6-type {129,133,134,135,136} tracker {$increment_tracker($tracker)} keep state