The historians fallacy and moral relativism.

I am going to put the Historians Fallacy from this website in here as reference

Description: Judging a person's decision in the light of new information not available at the time.

Logical Form:
Claim X was made in the past.
Those who made the claim did not take into consideration Y, which was not available to them at the time.
Therefore, this was a foolish claim.

Me Again...

I often get into conversations about historical figures, and practically all historical figures did things which were common in their time but are now considered extremely immoral. When I say that X was a common practice in the past I usually get an angry reply. X HAS ALWAYS BEEN WRONG!

Take this for example. It used to be a common practice to smoke with a baby in the car. Imagine this scenario. A man is running for president who is old enough to have lived during this time when smoking in a car with a baby was considered normal behavior. Then he is judged as a terrible father in the present for this. Is this the historians fallacy?

This might be more of a question of morality than logic or fallacies. Let's take your smoking example. A long time ago, when this was common, people simply did not know the harm they were causing their children, yet harm was being caused. So how are we defining "bad parent?" Based on the outcome (the health of the child) or the intent (the parent's efforts)? We can say that it was objectively bad for the child, but we can't say with the same confidence that the parents were objectively bad.

What about slavery? It is difficult to imagine that people in the past did not feel guilt or know that there was something morally wrong with enslaving humans. I say this because human empathy does not change with culture—it is a biological constant. Culture and norms can desensitize our empathy, but only to a point. I see a parallel today with our treatment of animals (mostly eating them and perhaps experimenting on them). I eat chicken and fish, yet I believe it is morally wrong to do so. I am confident that when lab grown meat takes over, eating animals will be seen in a similar light to slavery. So can someone judge me in 2050 as a terrible person for eating meat in 2018? They can, but because they have access to abundant, cheap, and delicious lab grown meat, they cannot accurately empathize with my situation which would reflect on their judgement of me. Here is the important conclusion: If I am lucky enough to be around in 2050, my eating meat in 2018 should NOT be judged the same as if I were to eat meat in 2050. The basis of the historian's fallacy is that it is fallacious to attempt to understand someone choices (moral or otherwise) without considering the information they had or did not have at the time.

As the good doctor explained this has less to with logic and more to do with cultural mores. If it was a fallacy of logic it might be better described as Chronological snobbery: A fallacy of distraction that accepts or rejects ideas solely based on its age.