Well AD&D fights against you if you are interested in that. I know, I was there!
I'm broadly with
@Mannahnin here.
I mean, if "modding" means
I make whatever changes I want to this with no expected change in the basic play experience then it would seem that no RPG can be modded, because
some change will affect the experience.
What I tend to feel is that "can be modded* means
this RPG presupposes that the GM will mediate or calibrate the experience, probably in real time, and so if the GM changes the rule system they will still be playing that mediation/calibration role, and hence the modifications won't reveal themselves because though they change the input, the output is still sitting on the other side of the GM.
And I think it's absolutely true that 4e does not assume that the GM needs to mediate or calibrate in real time in order to deliver the play experience. It's not an RPG in which
the game is (or is close to being)
the GM.
Which takes me back to these two propositions being different ones:
*The GM is discouraged from using pre-conceived fiction to shut down mechanically permissible action declarations;
*The fiction doesn't matter.
The decentring of the GM as curator of the play experience seems to me to be the most fundamental reason why many RPGers did and do not like 4e D&D.