Happened across a possible typo in documentation specifying FE80::/8 as the link-local subnet for IPv6 at https://docs.opnsense.org/manual/ipv6.html#router-advertisement-ra (https://docs.opnsense.org/manual/ipv6.html#router-advertisement-ra) and according to everything I can find (https://datatracker.ietf.org/doc/html/rfc4291#section-2.4), the IPv6 Link-Local unicast subnet is FE80::/10.
I have not checked any working code to see if this value made its way into development or production code, but wanted to share the finding as it would appear to be a technical inaccuracy so we can start by updating the documentation.
Extra:
There is a "/8" but it is found in FF00::/8 which is IPv6 Multicast and not IPv6 Link-Local unicast (FE80 != FF00).