There is nothing wrong with open sights if you have good vision and it is no problem to shoot 500 meters with them. The sights are not the problem at all until you get old.
It depends on you alone and if you can hit deer at 100 or 50 or only 25, those should be your limits. Ethical hunting is first based on what you can do.
When I was young I shot open sights better then a scope and shot tiny targets at 300 and 400 yards from Creedmore position with a revolver. Now I don't know if they are on the gun sometimes. I have gone to the Ultra Dot for deer.
But the same question holds, how far can you hit reliably?
No matter what, even if I can shoot deer at 100 with a lot of practice, I still like them as close as I can get them. I much prefer 20 yards!
If you have good vision, it is not the sights that limit range, it is you and how much practice you have. Base distance on that alone.