Note that the actual sequence is 3 characters (newline, tilde, question mark). If you try ~? after anything except a newline, ssh won't intercept it (this is true for all ssh tilde escapes).

Telnet had something similar.

This is of course specifically how to reveal these features in the OpenSSH client. If you're using another client (e.g. PuTTY) there's a different way to get to these features (click the menu).