Fighters that had a dramatic decline , and which fight/ fights, showed you they were no longer the fighter they had been? Roy Jones Jr is a obvious example, Tarver 1 and 2 .
Tom Sharkey, Joe Joyce,Joe Frazier. Sharkey v Munroe,Ruhlin 2 ,and Fitz 2. Joyce v Zhang 2,Chisora, and Suslenkov. Frazier ,nearly all post FOTC.
Jarrett Hurd, Joe Frazier was still a formidable force going into the Thrilla in Manila, but had absolutely nothing left after.
Aaron Pryor and Jeff Chandler faded dramatically at the very end of their careers. Matthew Saad Muhammad is an obvious one where he had declined badly by the time he fought Dwight Muhammad Qawi due to his endless punishment and back to back wars that took a lot out of him.
Josh Taylor is a good recent candidate, went from unifying the entire division to absolutely stinking the place out vs Catterall in the space of about 6 months. Jack is a good boxer, don't get me wrong, but Taylor looked woeful.
Donald Curry is a big example of someone who was briefly seen as a top P4P fighter until one fight changed everything, in this case his fight with Lloyd Honeyghan. Never could capture the same form that he had since. Livingstone Bramble also fell off after being stopped by Edwin Rosario.
Cleveland Williams was never the same after being shot. Bowe was essentially done after those Golota fights. Tyson wasn't the same after the Holyfield losses. Gvozdyk was a promising young fighter but was never the same after the Beterbiev fight. Rigo and Walters were pretty much finished after the Loma losses.
Jones after the HW win had just enough left the in the tank to beat Tarver... And was done at top level after that.
Bermane Stiverne is a decent shout. Easy too look back on him as crap now, but stopping Arreola how he did was impressive, and he took Wilder the distance.. He went from that to someone who looked like they didn't belong in world level bouts real quick.
Hello, Fergy. Evander Holyfield suddenly declined after Bowe 2 in 1993. But! Then he seemed to recover in 1996...
Paul Banke completely lost any semblance of being a world level fighter after his third war with Zaragoza. He lost to absolute nobodies after that fight, he was a completely spent force.