SF_11.0, SF_11.1, SF_11.2, SF_12.0, SF_12.1, SF_12.2, SF_13.0, SF_13.1, SF_13.2, SF_13.3, SF_14.0, SF_14.1, SF_14.2, SF_14.3 - https://docs.sitefinity.com/encrypt-sensitive-data-in-configurations

Encryption_web_config.xml

<appSettings>
  ...
   <add key="sfconfig:defaultNotifProfileSmtpUserName" value="testUser@123!"/>
   <add key="sfconfig:defaultNotifProfileSmtpPass" value="P@$$w0rd!"/>
  ...
</appSettings>