If you press Y, you'll start typing in team chat, so only people in your current team will be able to read it... So if you joined a /team, or if you're in Spectators.
If you join neither, the people that also haven't joined will be able to read what you say freely. It's more-so an old mechanic from when it was PVP focused, in vanilla Teeworlds. (Team Deathmatch, CTF) (edited)