Any way to send just "\n" in Telnet?

You can negotiate binary mode. Once in this mode you cannot leave it. Negotiation means the telnet client will send a special byte sequence to the server, which you will have to ignore if you are not implementing the protocol.

Subsequent data is sent unchanged, in line mode. Client:

$ telnet localhost 1234
Connected to localhost.
Escape character is '^]'.
^]
telnet> set binary
Negotiating binary mode with remote host.
hi
  ^]
telnet> quit

and server

$ nc -l 1234 |  xxd -c 1 
00000000: ff  .
00000001: fd  .
00000002: 00  .
00000003: ff  .
00000004: fb  .
00000005: 00  .
00000006: 68  h
00000007: 69  i
00000008: 0a  .

Your telnet client may have an option to start off in binary mode, or you can put an entry in ~/.telnetrc

localhost
 set binary

You can apply the binary mode independently in each direction, so you might prefer set outbinary.