Just to add something else to the discussion, I read that some said that it is not really unfair or a big problem because you can easily calculate the health of the opponent after his fight, but the problem is that we can't always see where the player comes from. How would I calculate his health if I was distracted or I was not able to see his fight (because of a wall, for example)?
Or even, if I see his fight, I could wrongly assume his state of health due to his armor or the weapon his opponent used. I can't see what enchantments they have (if they have enchantments) or even I might not be experienced enough to figure it out. I do not see the sense in saying that this is solved with knowing how to calculate (which I consider the same as saying, be better playing).
It seems to me that being able to see the health of others is really unfair.
Personally I would look for a solution that does not require punishing those who use them (because I think it would be very difficult to detect it and because those who had already been using it would suddenly be banned by Sentinel without warning)
My preferred solution would be to find a way to stop the damage indicator from working. But the simplest solution that I see to this (although it is not my favorite) is that the same games show to all the players the health of others. Maybe with the option to disable this for whoever wants it, but I don't think it will be very used because if everyone has this on, I probably don't want to be intentionally disadvantaged.