> Just out of curiosity, what are notable examples of people saying “we should judge people by the standards of their time” in a reasonable way?

Do you consider eating meat at a restaurant to be a morally acceptable thing to do?

The meat you consume there is likely factory grown, which means that the animal lived in total misery until it was brutally murdered under extreme duress.

It's highly likely that people will consider treating animals like that to be immoral at some point. If you don't accept your quote as truth you'll also have to accept that your descends will consider you (and everyone else in our society) monstrous.