Should be available.
Every single other MMO with cross-server technology found a way to do it.

If it's a design decision, I heartily disagree with it. If they think people will harass others through /tell, so they disabled /tells in a dungeon, that's a bogus excuse.
It doesn't stop people from harassing at all - they just do it in party chat. You're not going to stop toxic players. That's what the blacklist is for--Which doesn't work cross-server?
Uh-oh.

Square Enix REALLY need to get themselves into the 21st century with their network infrastructure. This is getting old.
I'd expect this from 1.0; but not ARR.