City of Heroes is one of the games that had a player base that wasn't necessary massive but was more than dedicated. Many of those fans believed the game to be dead since NCSoft shut the servers down but this was not the case, due to SCORE.
SCORE is an acronym for Secret Cabal of Reverse Engineeres, a name that a group of people gave themselves upon receiving the City of Heroes character data from 2012. Just like the "cabal" part of the name implies, the existence of both SCORE and their server was kept a strict secret.
According to Leandro Pardini, a member of SCORE, he was among the players who started saving character data on their PCs just before City of Heroes shut down. Once it did, they started developing a private server, recreating the beloved game.
Six months into development, he was contacted by an anonymous person who claimed they had all character data from NCSoft's servers and eventually sent them to Pardini over encrypted networks giving him and the team all the necessary tools to begin a new era with City of Heroes.
Pardini happened to be among the players who attempted to pull off a similar stunt with Tabula Rasa though after NCSoft shut that game's servers down. Soon after their exploits were made public, NCSoft's legal team issued a cease and desist letter, forcing them to abandon their work, effectively putting an end to Tabula Rasa's private servers.
Not wanting to repeat the same mistake with City of Heroes, Pardini and the team slowly grew the game's community through family, friends and people they trusted, eventually reaching about 3,000 players according to YouTuber named Destroyer Stroyer, who exposed the server's existence.
In his own words, Destroyer Stroyer was invited to a secret private server for City of Heroes a couple of years back after posting some nostalgic pictures on Reddit. This turned out to be SCORE, and after playing for a while he apparently wanted to make its presence known to the public.
It appears that the SCORE team attempted to keep the server secret through all of this, but Destroyer Stroyer eventually posted several YouTube videos on it.
According to PC Gamer's communications with Leandro Pardini, he received death threats for not making the server public, despite obviously having solid reasons not to do so. Furthermore, he stated SCORE will remain behind closed doors in order not to expose any more members of the team to similar treatment but the source code will eventually be released to the public.