185. Agatha Christie 

That's a feature of the production and the screenplay to maximise their viewer appeal. I've seen some recently which are darker than the books themselves.

Podbean AFAIK

Literary trope, or correct observation ?

@@DaveS859 'Correct' observation, I suppose if you happen to be an arch conservative who doesn't believe in social equality.

The fact that someone makes correct observations about a period about which they're writing doesn't imply that they hold 'beliefs' about anything and neither does it matter if they did. The world that Christie recounts is exactly as it was. It applied (and still does) to 'working class' people as well who'd have been lamenting lack of respect from people they regarded as beneath them. We still get endless complaints from middle aged and old people today that young people don't 'respect their elders'. We've become so used to literary works containing political propaganda nowadays that some ignorant types judge the whole work on whether it's Woke enough. In any case there's never going to be 'social equality'. No one can dictate to each of us how we view the status of someone else. We each have our differing values and prejudices, and we each have to find our own place in the community.