The way this was done was a major headache inducer, requiring reconstruction of the function each time the value was changed and in general made actor damage a major hassle.
There was a DECORATE wrapper to mimic the original behavior but this looked quite broken because it completely ignored the different semantics of both damage calculation types.
It also made it impossible to determine if damage was a function or a value.
This accessor has been reverted to what it should be, only returning the constant, which now is -1 for a damage function. I am sorry if this may break the odd mod out but a quick look over some DECORATE-heavy stuff showed that this was never combined in any of them so that accessing 'damage' in DECORATE code depended on an actual damage function.
To get proper damage, a future commit will add a DECORATE function which calls AActor::GetMissileDamage.