Buried in the FAQ and not in the changelog for some reason:- As of 0.68, PuTTY will no longer fall back to SSH-1 if the server doesn't appear to support SSH-2;

Buried in the FAQ and not in the changelog for some reason:- As of 0.68, PuTTY will no longer fall back to SSH-1 if the server doesn't appear to support SSH-2; you must explicitly ask for SSH-1.