When Did Heavyweight Boxing Become Modern?

Concur ^^^^. But there are single exceptions in fighters such a Jack Johnson, who most boxing experts recognize as one of the greatest defensive fighters of all all time.

FYI: John L. Sullivan, gets 2nd wind after 44 rds....wins in 75th by t.k.o., not considered "modern".
On July 8, 1889, an estimated 3000 spectators boarded special trains for the secret location, which turned out to be Richburg, a town just south of Hattiesburg, Mississippi. The fight began at 10:30, and it looked as if Sullivan was going to lose, especially after he vomited during the 44th round. But the champion got his second wind after that, and Kilrain's manager finally threw in the towel after the 75th round.

 

There's no such thing as "modern style" in boxing.
Heavyweights are a law to themselves anyway. Most of them look clumsy and crude at first glance, whatever the era.

 

Especially with the (lol) gloves made of wrapped leather thongs I notice the guy throwing the straight punch seems to have dropped his right hand off his chin ... nnaughty, naughty!

 

I think Dempsey and Tunney look very modern.

 

When did it become modern? When Corbett beat Sullivan under the Marquis de Queensbury rules.

 

The fact that no two people gave the same answer probably indicates that there is no obvious shift from old to modern.

 

Good point. There is a short silent film of John L. with some of his younger contemporary fighters in suits at a "reunion", lightly 'sparring' with each other, and John L. is throwing some "mock" uppercuts and body shots that would make Tyson pay attention to his technique. As they say even today: "All heavyweights carry power".