PuTTY proxy password in registry
Plain text password in registry
If you set a proxy password in PuTTY and save the session settings, you will save the proxy password in clear text under the name of the profile in the registry.
The password can be seen here.
The user should decide for himself whether that is unacceptable or not. In my opinion, no matter how insignificant an application / option is, a password should never be in plain text anywhere.