Were you referring to people like Andy Hug of Kyokushinkai?
I believe he suffered from an illness, but in general I suppose having a full-contact punching and kicking bouts with gigantic men isn't very good for your long-term health anyway. Especially in Kyokushin where they have (had?) no weight classes.
I actually don't think it is the direct damage from fighting that gets them but more the inability to stop and scale your commitment to what you can and can no longer do.
Direct damage from combat sports can catch up to you eventually. CTE is pretty serious, but unfortunately you probably don't realize the extent of that damage until it's far too late.
I believe he suffered from an illness, but in general I suppose having a full-contact punching and kicking bouts with gigantic men isn't very good for your long-term health anyway. Especially in Kyokushin where they have (had?) no weight classes.