In more recent times, NetBSD has added OpenBSD's version of nc to the base system. One could thus argue that there is a TCP client "installed on every system".

In more recent times, NetBSD has added OpenBSD's version of nc to the base system. One could thus argue that there is a TCP client "installed on every system". For me, what is more important is an installed copy of GCC. I have yet to use a UNIX-like system that did not have the needed networking functions to create a basic TCP client.