It's not necessarily that the data was obsolete. It was Toynbee's use of it. Toynbee thought there was a definite pattern in history of civilizations rising, becoming decadent, and falling apart, and that there were also common factors in the process. Gibbon had famously played something of the same game, in his Decline and Fall of the Roman Empire, but Toynbee went world-wide, and covered 23 different "civilizations".

There are all sorts of problems with this. First, there's something very useless about the term "civilization": it implies progress, and high-mindedness, as opposed to , you know, cavemen digging in the dirt and hitting each other with clubs, and Toynbee's structure therefore tended to cast human history as high-minded progressive people vs dirty cavemen with clubs. Things don't always work that way: in the Roman Empire, for example, those barbarians really liked a lot of the civilized things the Empire had, interacted with it in many mutually-beneficent ways. So, to call it a "decline" is somewhat like saying a bookstore has "declined" when a new owner buys it. Second, there's Toynbee's very wide net, trying to show all those different "civilizations" had common dynamics, structures, problems. If you want to show that two or three "civilizations" had common structural problems, it might not be hard, but in trying to do that with 23, he ended up pounding some square pegs into round holes, just to make them fit, saying, for example, that all states have a Time of Troubles followed by a Universal State with a Universal Church, supported by internal proletariats and opposed by external proletariats. Even in Toynbee's time, this had problems, but in the recent discoveries about New World states, like the Maya and the Inca, there aren't even pegs for some of those square holes.

But there's something immensely appealing about the game of Decline-And-Fall. Before Toynbee there were others, like Gibbon, and Oswald Spengler, and there's tons of dystopian futurist fiction being written now. In times of crisis people will want to play it, want to show that there are ravening hordes at the gates of our beautiful city who want to tear it to bits ( Nial Ferguson comes to mind). Lots more could be written about that, of course, but I would only say that it can be simpler to think of fighting ravening hordes than getting along with the new owner of a bookstore.

However, though no one much has time to try to read Toynbee anymore, you have to give him some credit. He worked immensely hard at his Study, and in it are some very astute observations and, above all, there's very good writing.

https://www.cambridge.org/core/journals/comparative-studies-in-society-and-history/article/return-of-civilizationand-of-arnold-toynbee/FE6F858900CBB1843DD7C0D3DD5BE360/core-reader