The short answer to what happened to Genera is that it's owned by a private holding company that bought up a bunch of Symbolics IP in the 1996 bankruptcy proceedings. They made one further release in 1998, and afaik haven't done any development on it since.

Presumably there's some price at which they'd be willing to either open-source it or sell it to someone who would put dev resources into it, but that hasn't happened.