The code may need to be adapted to evolving environments. Security bugs might be discovered. In this case I guess "being maintained" just means that someone will react to such events if they happen.
Couldn't programmers from major GNU/Linux distributions be signed up for this duty? They compile new OSes and if they want man pages (heck, who doesn't want man pages?) then it's also up to them to make it work in the new OS. Security bugs can be forwarded to those people by Gnu (assuming they support and host the project).
This may not work for everything, they're not going to maintain any old system, but things that we want to keep around but for which there are not necessarily dedicated maintainers to keep it up...
