The client doesn't always have that information (especially if you're doing Unixy things and piping through netcat), but when it knows, it should certainly indicate that, which is why there's TCP_NODELAY and friends; I couldn't find out exactly when TCP_NODELAY showed up, but old BSD man pages [1] have the option and a date in 1986. In the absence of explicit information from the client, I think Nagle's algorithm is a decent heuristic.
[1] https://www.freebsd.org/cgi/man.cgi?query=tcp&apropos=0&sekt...