Social MU*s usually have a keepalive option that does some silent character exchange in the background with the client every so often to keep the connection open.
Parent is most likely referring to the telnet GOAHEAD character and its related SUPPRESS GOAHEAD protocol: https://tools.ietf.org/html/rfc858
I am the author of a MUD client for iOS[0] that, like most other clients, will capture and silently discard the GOAHEAD character. GOAHEAD is most common on MUSHes but I've seen them in a few MUDs as well.