If you use a vanilla 1.16.5 client and try to connect to a vanilla 1.17.1 server, the server will respond with the message "Outdated server! I'm still on 1.17.1" when it is the client that is outdated.
In the past, the server would respond with an accurate message. For example, if I connect to a 1.16.5 server from 1.16.3, I am disconnected with the message "Outdated client! Please use 1.16.5"