Again you're talking about things that may not be as simple as you think if you've not seen the code they use.

Just because it exists for trial characters doesn't mean that it's something that can be easily put onto non-trial characters.

They'd have to add the option to the NPC. They'd have to make sure there were plenty of warnings that no exp gain can be granted while on that class.

They'd also have to likely put a reminder on log in for players who logged out on the exp stop class or swap to the class to remind players that the class is on no exp gain to avoid the countless "bug" threads from players who forgot they put the class on no exp gain.

They'd still likely get reports from players who just clicked past those pop ups without actually reading them.

They'd also have to very rigorously test it to make sure that there are no errors with it. Because the last thing they need is players getting stuck with no exp gain on and not being able to swap off of it.

It's still something that I wouldn't hold your breath on being added any time soon given the fact that it appears to be something not many people want.

That all being said, I do hope that one day they do add in something like this for players like yourself, even if it is just a exp gain reduction item as opposed to a completely stopping of exp gain. I just don't feel that this is something that's high on their priority list.