From Quake Wiki

Builtin definitions:

void(float to, string s) WriteUnterminatedString = #456;

Like WriteString, but does not write a terminating 0 after the string. This means you can include things like a player's netname in the middle of a string sent over the network. Just be sure to end it up with either a call to WriteString (which includes the trailing 0) or WriteByte(0) to terminate it yourself.

A historical note: this extension was suggested by FrikaC years ago, more recently Shadowalker has been badmouthing LordHavoc and Spike for stealing 'his' extension writestring2 which does exactly the same thing but uses a different builtin number and name and extension string, this argument hinges on the idea that it was his idea in the first place, which is incorrect as FrikaC first suggested it and used a rough equivilant of it in his FrikBot mod years ago involving WriteByte calls on each character.