I have slowly been working on a MUSH, which is an online, multi-player, text-based social game. The game Zork (1977) is probably the most popular game in this style, although it is not multi-player. I decided to use PennMUSH as the server distro since it seems to have the widest support.
MUSH servers open a raw TCP socket for incoming connections and most sessions occur over plaintext. There are a handful of MUSH clients that players can use but a simple TELNET connection also works. MUSH servers understand enough of the TELNET protocol to refuse option negotiation.
Rather than expose the MUSH server to the open internet, I decided to create restricted shell accounts for users and have them connect to the MUSH via localhost instead. This drastically reduces the accessibility of the MUSH and would likely deter prospective players but it works for our current user base.
Despite the huge number of active MUDs on the internet, forcing users through SSH still seems safer than exposing the MUSH service. I'm using AWS security groups on the MUSH host to prevent external access to that port.
Note: After some testing, I have walked this back and decided to just open it to the internet after the initial release. Shell accounts will still be an option. See this page for some details.