Legally speaking, City of X exists in a bit of a gray area. On the one hand it's protected by contract law in the form of the EULA we all clicked off on to play the game. And also by Digital Millennium Copyright Act, which makes reverse engineering it without permission technically illegal. (DMCA is under challenge all the time, which is why I use the T word)
On the other hand, the clients on the internet are perfectly legal to possess as they were freely distributed by the company, and I don't remember the EULA specifying they be deleted if the game were discontinued. those files can no more be made illegal than a free AOL CD can. And also, lots of City of Heroes material exists on the internet, including the CityofHeroes.ca website, billions of screen captures of the environment, copyrighted characters, interface, etc... If NCSoft wants to protect its copyrights, it has to tred carefully on the existence of all of these items.
Fair Use law indicates that since the client software was freely distributed, that we are all still free to use it as we see fit so long as those uses are not-for-profit. The problem is that the game in its native state requires an active game server, and those were NOT given out freely by NCSoft, so creating one would either require DMCA-violating reverse engineering (yes this is going on) or direct theft of the code from NCSoft. in other words, both illegal.
on the yet another hand, there is such a thing as abandoned property law. and it can be applied to electronics and data. after a certain amount of time, if a company, publisher, developer, distributor, etc, has done nothing with a product that is already in the public sphere... and refuses to provide active support, warranty service, etc, on it... then that product can be considered abandoned, and thus public domain. There is an entire market out there for abandonware videogames... and right now, approaching one year after closing, the clock is ticking on the abandonware timer.
In general, the EULA is effective in law right this very moment, but since the isn't an active game, it's legal force is very limited. personally, I don't have the money to hire lawyers to sort out the more finite legal minutia, but a good lawyer could make a case that the game is either abandoned, or viable for fair-use-protection on private servers so long as no money at all is involved... but don't quote me on that.