I agree, I've seen at least one old boxer randomly come to the gym to hit the bags, I couldn't understand him. Our coach inferred that's what happens when you take fights out of your class or take fights solely for money without caring about a loss. I don't doubt it happens but again as a casual amateur I don't think its something you have to worry about. Another thing a lot of people who don't box may not realize, is that if you get rocked, take a knee. The fight is paused and the coach can make sure you're good to proceed, it's like tapping in bjj, you do it to avoid injury. Like I said boxing isn't without risk but brain injury shouldn't stop you from trying it out. Most people quit before they're even ready to spar, the conditioning is hard enough and its worth experiencing.The problems don't manifest in the gym or the days and weeks that follow. Years down the line can be an entirely different story.
I run into guys from time to time that I knew from boxing. Haven't seen them in twenty years or more. Some of them are just fine, but others are not, they can hardly be understood when they speak. Makes me want to cry sometimes.
Don't get hit in the head too much. Especially if you box.