1) The past is not indicative of performance on the show. Hence using the past to say why someone is a good or bad contestant is problematic. The focus should be on actions in the house.
2) The past is not indicative of behavior in the house. Sweethearts in real life have turned bitches in the house and real life bitches have been sweethearts. Using the past to explain a behavior, characterize people as good or bad is again problematic. The focus should again be on actual behavior in the house.
3) Professional decisions, personal life choices are all unique and circumstantial to an individual. They should not be used to demean or discredit people. It should also not be used as a means to unfairly characterize and stereotype a person. There is also a line between past and present. People do change according to circumstances and deserve a fair chance.
So Sana Khan has had some provocative ads. She has acted in some provocative films that were somewhat soft core. That doesn't make her any less human or characterless. She is just a girl who has dreams of stardom, she made some choices to get to the goals. Perhaps we wouldn't make those choices, but each and everyone of us makes difficult choices to get where we want to be. None of them make us morally superior or inferior to each other.
The problem with bringing Sana's past was not the rule, but how people brought up her past. The intent majority of the time was to negatively stereotype and assassinate her character. The intent majority of the time was to question her morality. No one ever presented her past as a nonchalant matter of fact statement as just part of who she is.
And when the problem at hand is her behavior in the house - why bring up her past. She was obviously insecure and jealous of Karishma. You can cite instances of her behavior in the house and point out her flaws. She did behave flirtatiously with Rajiv, Vishal and Aashka. Point out those behaviors and deem why it is unethical.
This is not just for Sana. I've been strongly against people using Sapna's divorces, her tattoos, her behaviors to negatively typecast her. I've voiced against people using Rajiv and Delnaaz's divorce as a framework to judge them. I'm against using Imam's homosexuality and flamboyance to define his character. I'm against taunting Urvashi over being a single mom. I was against Sunny Leone being judged solely as a po*n star. I was against Juhi and Amar being perceived as good over their past happily married personas.
In the end it boils down to how and why you bring something up and what your intent is. I've brought up past issues often times, but not inflamed fans or haters.