I genuinenly don't think that the advantage a Damage Indicator can give you is really that meaningful for it to be disallowed. There is more consequences and difficulties to this than benefits. Yes, you might somewhat decrease the amount of people who use it, just because some might actually want to follow the rules. But this is such a widespread mod that so many people will find no reason to stop using them, especially because there is no way to tell wether someone is using it or not. Whatever evidence you might get of it, there will always be the doubt of wether the person is using it or not. Nothing can prove it.
It would just be yet another rule that won't be enforced, basically for decoration. Perhaps discouraging the use of it is fine, but don't prohibit it since there's no methods to tell who is using a Damage Indicator. And it's not like it's as noticeable of an advantage as having reach or anti kb. Not to mention that people would start accusating each other because of using it, with no way to prove it. Big no from me.