The recent book "Capital in the 21st Century" by Thomas Piketty provides a lot of history on changes in the rate of return on capital over long stretches of history. In the early to middle 20th century, a great deal of capital was destroyed in the First and Second World Wars, and 'sound money' that was generally non-inflationary and usually no-risk 5% return on government bonds was replaced with 'fiat money' with floating interest rates on government bonds. (Note how the wealth of Jane Austen's aristocrats was generally represented as an 'income' rather than a quantity of wealth, as the ~5% return on capital invested in government bonds was so consistent that their income never varied year-over-year: https://www.pbs.org/newshour/nation/how-pikettys-inequality-theory-explains-mr-darcys-wealth)

As a consequence, the mid-20th century was a period of 'euthanasia of the rentier', in Keynes' memorable phrase (https://krugman.blogs.nytimes.com/2014/01/22/the-euthanasia-of-the-rentier/). Also, the rate of return on capital was lower than the rate of growth, or, in Piketty's summary, r < g. Per Piketty, in the interview above:

The wars had reduced the very sharp inequalities of wealth of the past. Very high growth rates in the ’50s, ’60s and ’70s made it easier to accumulate wealth and, for the first time in history, there was really significantly higher upward mobility.

The unusually high level of upward mobility during the post-war period was greatly facilitated by the large growth rate of labor income during these decades. But starting around 1980, we had a growth slowdown due to the fact that the post-war recovery was over and we returned to a productivity growth rate of around 1 to 1.5 percent — closer to the pre-World War I productivity growth rate.

Thus, in recent decades the return on labor, or at least enterprise, was higher than the return of sitting on one's ass. A lot of aristocratic bums got jobs, and today many wealthy people wish to at least pretend to earn a living through work as many of their immediate predecessors did.

As we are again returning to an age of great concentration of capital and slow growth in the industrial world, we may well again enter an age of aristocratic leisure.

It seems from historical fiction (eg Downton Abbey and similar) that very wealthy people in days past were content with just being wealthy owners of capital and not really “doing” anything. Their children would be given general education that would give them the social skills necessary to be aristocrats as but weren’t really expected to go to work as an engineer, doctor, teacher, etc. They were just content to be wealthy and preserve the family’s source of wealth so they could continue living a life that didn’t involve work. Work, even white collar work, seemed to be considered undignified by the aristocracy.

Sort of yes, sort of no. It's a bit more complicated. For one thing, education and expectations went farther than that in general, particularly for male children, from the eighteenth century on. Attending one of the right schools and universities was quite important - Eton, Westminster, Winchester; Oxford and Cambridge. As the schools were public, they would theoretically mingle with those beneath them (but not *too* beneath them) so as to better understand the people they would be governing, if they were older sons, since aristocrats inherited a seat in the House of Lords with their titles; they also mingled with each other, creating a social network that would be important for the rest of their lives. The classical curricula these schools taught also created shared sets of cultural references and values, and while they weren't really that *useful*, they were more rigorous than classes in etiquette and deportment.

Younger sons did not inherit titles or much money, and so did have to "do" something other than just preserving the family's wealth. Traditionally, they went into the House of Commons (in boroughs where their families lived, and/or rotten boroughs close by that could be easily won), the clergy (where they could eventually hold a wealthy bishopric relatively on par with a title), the army or navy (where promotions had to be paid for, and the wealthy and connected could rise quickly to a high position), or the diplomatic service (ambassadors to other courts, most importantly Madrid, Vienna, and Versailles). All of these jobs were, when you get down to it, more about contributing to the aristocratic control of the country than necessarily doing some kind of meaningful and useful work, so I think this does count as a "nominal occupation" in the same vein as the Bluth sons of the modern world who actually work for or are just given salaries from their family's businesses.

However, now it seems that the ultra wealthy are expected to “do” something, at least nominally, even if it is to have a fake job at the family business or run a money losing startup, or work for a non profit. If you look at the Forbes 500, most of the people who inherited wealth still “do” something.

Now, here's the other part of the picture. I would suggest that what you are seeing instead is a change in which social class has the most money and direct power. In the eighteenth century, it was unquestionably the aristocracy in Britain: the House of Lords was more powerful in Parliament, peers held most important positions in government outside of that, and titled families held most of the nation's wealth. During the nineteenth century and the Industrial Revolution, the upper middle class swelled with factory owners, bankers, merchants, etc. who accumulated vast fortunes and were happy to pass their businesses down to their sons, rather than buying titles and turning their money into investments that would provide regular incomes. At the same time, the progressive/liberal Whig party gained power and eventually passed a number of reform bills in the 1830s that set the country moving toward universal male suffrage, which made it more difficult for aristocratic families to swing elections to give their younger sons seats, and which eventually led to the supremacy of the House of Commons over the House of Lords. All in all, a background or even current position in trade became much less shameful and less of a bar to power, and the rich could retain and build on their companies while still being socially acceptable, allowing them to make more money, potentially, than an aristocrat whose income came from investments in the government funds and from rents paid by tenants on their lands. If you look at the Forbes list of billionaires for 2018, you can see that the world's top earners have come from money, but also have real positions in business - they started companies, or ran companies their fathers started - which fits in that mold. For nineteenth, twentieth, and twenty-first century industrialists, being productive and making money was/is a virtue and being completely idle a sin.

Now, I have to throw a bit of a spanner in the works and point out something here that your question unintentionally misses: women exist, and have always been held to a different standard. In the eighteenth century, aristocratic women did not sit in the House of Lords or take up a position in the navy - they married within their social network and at most became political hostesses to further their families' goals. In the nineteenth century, wealthy industrialists' daughters didn't continue their fathers' businesses - they married other industrialists' sons to cement or create business alliances, or married into the aristocracy to cement their social positions. Today, many young wealthy women may do all kinds of important, socially-conscious, or nominal work, and always have degrees from top schools, but it's still quite common for women in elite society to drop work outside the home once they're married and have children. In that sense, some things never change.

Follow up: Has this apparent attitude change been specific to the U.S./the West, or is it true around the world? Are there places or communities that still promote a culture of leisure among aristocrats?