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Modern airliners generally have lower cruise speeds than those of comparable aircraft from 40 years ago (the venerable Boeing 737 seems to be something of an exception; it has always had a more leisurely cruise speed).

Airlines had to respond much faster to the oil crisis than manufacturers could - to what extent were airlines able to economise on fuel consumption by flying more slowly, and what sort of margin was available to them at cruise (and by extension, what sort of margin is available in modern aircraft, should economic factors drive operators to seek lower fuel consumption by flying more slowly)?

As a French who grew up in the Caribbean, I can tell that flights are getting longer. A flight between France and my island that took 7.5 hours 15 years ago is almost 9 hours long today.

This post published by the MIT’s School of Engineering says (emphasis mine):

Specified cruising speeds for commercial airliners today range between about 480 and 510 knots, compared to 525 knots for the Boeing 707, a mainstay of 1960s jet travel. Why? “The main issue is fuel economy,” says Aeronautics and Astronautics professor Mark Drela. “Going faster eats more fuel per passenger-mile. This is especially true with the newer ‘high-bypass’ jet engines with their large-diameter front fans.”

Today's high bypass engines are more efficient at low speed:

Observant fliers can easily spot these engines, with air intakes nearly 10 feet across, especially on newer long-range two-engine jetliners. Older engines had intakes that were less than half as wide and moved less air at higher speeds; high-bypass engines achieve the same thrust with more air at lower speed by routing most of the air (up to 93 percent in the newest designs) around the engine’s turbine instead of through it. “Their efficiency peaks are at lower speeds, which causes airplane builders to favor a somewhat slower aircraft”

This article also states that slower aircraft's structures are cheaper to build:

“A slower airplane can also have less wing sweep, which makes it smaller, lighter and hence less expensive.” The 707’s wing sweep was 35 degrees, while the current 777’s is 31.6 degrees.

I have only my own anecdotal evidence. I was flying domestically & internationally quite extensively during the 1970s, and particularly the period spanning the oil crisis.

Although the impact of the 747 on mass air travel had begun in some markets, most aircraft were still legacy assets, often belonging to government owned national carriers that were still run by pilots, engineers & bureaucrats, not accountants & MBAs. It was very common on long international flights to have aircraft that were only half-full, if that; my first flights between Australia & the US or Europe were spent stretched out over three seats, this was considered to be commonplace at the time.

The airlines reacted to the oil crisis by immediately cutting the number of flights, & getting rid of a lot of bulk-buy cheap ticket deals to travel agents. In many cases it was a physical shortage of fuel as well as the increased cost that excused what led to sudden huge increases in queues at airports that were still just glorified empty hangars that had never dealt with crowds.

Long distance aircraft were constrained much more by having to stop & refuel much more often than they do now; what is today a single hop from Sydney to DFW, was then Sydney to Fiji, then Hawaii, then LA, then domestic.

I don't recall the aircraft going much slower, or at a different altitude (not sure if that was practical), but there may have been some adjustments with somewhat less flexible legacy aircraft such as the 707s & DC8s. They were already flying as efficiently as they could, just to avoid having to land. With fewer aircraft flying, they were now more likely to be operating closer to their maximum weight, which constrained performance.

Early 747s were more economical to start with, but when they were first introduced they had far fewer seats than post-oil crisis, when they were quickly reconfigured to get very crowded. Pre-crisis you preferred a new 747 for its space; post-crisis you hoped for a DC8 because they couldn't rob you of seat width.

With domestic air travel, again, they just cut flights & charged like wounded bulls. There were many non-flying areas in their operations that were hugely inefficient that could be made less costly, & these were all overhauled. Political lobbying became much more important in order to reduce technical & employment regulatory "costs". Refuelling at lower-cost airports & tankering changed scheduling.

TLDR, yes, they probably slowed aircraft slightly, but that wasn't nearly enough - instead the airlines cut flights massively & ordered smaller seats for their 747s. It took years for airlines to recover, many just went bankrupt.