Question SkillListener behaviour after 200m xp?

No because the experience you're gaining is no longer applying to your account.

 

Honestly why would you keep doing a skill you're 200m exp in?

 

This is probably a one in a million case, to continue botting at 200m exp. You either don't know you have 200m exp, or you're stupid. I understand the concern to avoid a unusable bot for certain people, but I think that population is so small, it can be neglected.

 

Even if it's highly unlikely, it should still be supported.

 

It depends.
If the cap is 200m, even if you "gain" experience past that point, it's not actually being gained. Think of it as being discarded instead I guess.
Imo the current implementation is the most logical one

 

What are the consequences of not tracking it past 200m?

 

Well first, there's the question of does the event trigger once you've reached cap? From what @Cloud said earlier it sounds like the experience is not applied to the account, so most probably not.
Second, since you're not actually gaining experience, wouldn't saying that you've gained exp be misleading/incorrect? 200m + 1m exp at 200m cap is 200m, not 201m.
Either way, I'm not opposed to it being tracked, but imo it not being tracked is the logical result.