EXPERIENCE boost from high level classes. Is it true

[Niwashi]

This boost essentially becomes permanent once you get any class to level fifty (since it's the current highest level).

It's basically always permanent. Taking your example of ARC 30 giving PUG a bonus until PUG reaches 30, as soon as PUG surpasses the experience of ARC, the bonus would simply transfer. Then instead of PUG continuing to get the bonus, ARC would have it instead, because now PUG is your highest class. You still have the bonus on all but one of your combat classes, it's just a matter of trading off which ones.