Maybe not quite up there with some of those already listed, but Khan getting demolished by his career rival probably deserves a mention.
Scott Quigg's final defeat to Jono Carroll where he was totally outclassed and should have been pulled out the fight much earlier than he was.
Hopkins getting knocked out through the ropes by Smith Jr was both an EXTREMELY humiliating and hilariously appropriate way to end his career. He was a guy that built his reputation demonstrating cageyness and durability while also showing utter contempt for white fighters. So for him to get laid out the way he did against that particular opponent (who he had handpicked) was quite memorable.
Don't know why this came to mind in particular, but the guy who Murat Gassiev almost decapitated with a left hook, Jordan Shimmell. Never fought again.
Leonard vs Camacho deserves a mention, Ray's chin looked absolutely gone. Also Hatton vs Senchenko, what a humiliation.
As above there has been many ( sadly ) but the unmerciful beating Ramos took from Carmona still haunts the historians, Mando paid an awful price for the years of parting, the picture of him being carried from the ring on a stretcher, with one arm flopped out to give the erie look of a corpse, was a sad indictment to all the backslappers and hangers on, that only hastened his pitiful demise, it should've been a stark warning to all young talented fighters that there is only one ending when you burn the candle at both ends, alas we know that it didn't. stay safe guys.
Floyd vs McGregor Tyson vs McBride SRL vs Camacho Recently Holyfields exhibition with Belfort was disgusting. Belfort should be ashamed of himself.
Grover Wiley never won another fight after this!! Chavez jnr knocked him out 2 years later to perhaps gain some revenge for the family...lol
Pernell Whitaker vs Carlos Bojorquez and Michael Spinks vs Mike Tyson. Whitaker is widely considered a top-3 all-time LW and Spinks comes up in discussions about all-time Light Heavyweights.
Any time Mayweather Jr does another exhibition I pray it's the last one...or that someone finally knocks him out. I really don't like these old fighters doing the exhibitions. Sometimes it works but mostly it's just sad.