Is it bad form to answer questions regarding “retired” games?
Absolutely... not! The purpose of this site is to get help for anything about gaming, no matter how old. You can talk about arcade machines that came out in 1977, and we'd still try to help with any problems you may have with them (even if the problem is as simple as a quarter being stuck in the slot!). Any games that are retired can be revived, if set up right; however there are some games which would require much work (even years) to bring up-to-date, especially if a required component has become obsolete or missing (i.e. a game server, in regards to City of Heroes), but this does not mean that it can never be done.
As long as the content of the answer was at its last time of being active correct, is it okay to answer such questions?
Seems logical to me, since the game is going to be old, anyway. However, if you're trying to incorporate an old game into new technology, you'll need to know how to get the game to adapt to the more-recent machine. It may even be possible to get games that require some kind of network to work on modern internet servers - take games played over bulletin-board systems (BBS), for example.
In the case of City of Heroes, since all the servers that supported this multi-player online game were shut down in 2012, the likelihood of this game working in its current state is near impossible unless one is willing to tackle the task of redirecting the game's sources to a working server they themselves have maintained. Since the game has been abandoned, it is unlikely the maintainers of the game would reopen it again. However, it has been done to some online games; i.e. the one at this link: mystonline.com