r23081 introduced kern.dummy oid as a semi ABI compat for kern.maxsockbuf that was moved to a new namespace. It never functioned as an alias of any kind and was just returning 0 unconditionally, hence it was probably provided to keep some 3rd party programmes happy about sysctl(3) not reporting an error because of non-existing oid. After nearly 23 years it seems reasonable to just hide it from sysctl(8) list not to cause unnecessary confusion as for its purpose.
Reported by: Antranig Vartanian <antranigv@freebsd.am>