Benny Leonard is the first guy from the early ages that I saw on film and thought fit the modern style so I came here to say this, but admittedly I haven’t watched him in some time with an eye to just how ‘modern’ his style was.
That name came to mind, especially after the gloved era emerged which one could argue made him the first.
Going by what little film we have, Joe Gans looks like he could adjust to modern rules pretty well. He started boxing in 1893. I'm not an expert of this era but it seems that in his day Gans's was compared to Dixon a lot. It seems like he was a less aggressive version of Dixon. If we assume these comparisons are fair then Dixon should fit well. Which means your first modern looking boxer started in 1886. However there is one problem. We have a film of Dixon (past prime and not an official match) and he looks awful. So I'll stick with my answer being Gans. I could very well be wrong.
I know this is slightly off topic on what the thread was supposed to be about but I find your dead end idea really interesting. Would you throw Fitzsimmons into the same category as guys like Canzoneri and Wilde? Who would you say is the most recent "dead-end"?
I would based upon what i've read, i'm not sure what to make of it in those few rounds we have on film. I think more recently you just have wrinkles, Ali, Jones, rather than genuine explorers. But that's natural and you see it in most aspects of life that are reasonably old or that aren't deep (for example brain surgery which has psychological and physical aspects).
Dempsey is often the answer to this type of question - and certainly, that answer is not without merit. It also helps that Jack was a HW, a division upon which many primarily focus, and we also have very good vision of Jack compared to others from the era. Personally, as I see it, taking him exactly as he was, Dempsey would still look pretty damn good in the mod. era. And the way he fought Jack in their bout, the old Boston Gob wouldn’t look too shabby either.
There is no modern style after a point and that had more to do with the gloves, no styles are more modern then any other, it’s all just varying levels of competence in my eyes. Everyone since the dawn of being punched in the head instinctively thought to cover there face when you’re being hit, that is a flaw. Those real old school guys with there hands down, leaning back (Look at Mayweather jnr) knew something a lot of guys today did not- Blocking and not slipping as a first line of defence means losing out on a lot of opportunities to make the difference. So circling back, looking over in general I’d say in terms of incompetent and flawed methodology overtaking old school knowledge I guess it would’ve began in the 80s-90s? Could be very wrong I don’t believe there wasn’t proper pros in those eras just that the overall population had begun to dwindle.