Okay, I found a few more boxing titles. The reviews are a bit mixed on some of these but I figured I would at least mention them.
World Boxing Manager (PC/Mac)
https://store.steampowered.com/app/563750/World_Boxing_Manager/
World Championship Boxing Manager (PC) - a classic game port
https://store.steampowered.com/app/1254860/World_Championship_Boxing_Manager/
Punch Club (PC/Mac) - more of an RPG/Strategy game
https://store.steampowered.com/app/394310/Punch_Club/
Title Bout Championship Boxing (PC/Mac/Linux/Android) - outdated but still has a small community of people playing it
http://www.titleboutboxing.com/cgi-bin/page?home
World of Mixed Martial Arts 5 (PC) - I know it's MMA not boxing, but I still have to mention it because of how detailed, in-depth and realistic it is. I've played hundreds of hours on it, mostly thanks to such a fantastic community with superb modding.
http://greydogsoftware.com/wmma-5/
If I track any more titles down worth mentioning, I'll let you know.