There's likely plenty of things today that I'm sure that centuries or even decades from now will be considered morally repugnant. Animal cruelty, particularly in the context of factory farming, might be frowned upon today, but may one day be seen as unspeakably horrific.

Hopefully people will still be able to read books about factory farming, though.

I hope so too. What becomes taboo or morally repugnant in the future is hard to speculate, so I guess the better examples are yet to manifest. But to elaborate further, maybe eating meat might be verboten in entertainment for kids? Of course this would include re-runs/re-prints/etc. of old media.

or books where characters eat meat.