Hi Simone,
I'm sorry our change to a stronger encryption with salt caused a problem for your custom code but it is not a bug and that method is not obsolete, it can be and is still used from our password reset page so it cannot be made private but the salt has to be concatenated with the password before passing it into that method. I will update the description on that method to indicate that the password salt should be concatenated with the password before passing it into this method.
These security updates were announced in the release notes for version 2.3.8.1
Best Regards,
Joe