You realise writing a game does require you to leave stuff ambiguous in order to create interest and tension. We think that many mages are sent away. This isn't the case, it just hasn't been revealed yet. It will likely have some interesting reveal such as a clan getting slaughtered by a ton of abominations in a future game. Or perhaps, the Elves do claim a land they can truly call home and that's only by the assistance of a clan full of mages. BioWare are leaving possibilities, not cold hard facts.
Some things are obviously supposed to be ambiguous or characters opinions, and others aren't.
Things like whether the Maker is real or the mage Templar conflict are ambiguous. Some characters express one opinion, other characters express different opinions.
In this instance: two separate mages state that they were booted from their clans and Vivienne states that Dalish clans do so as if it is common knowledge. There are no characters who claim this is a rare or unusual occurrence, and everyone seems to think it is fairly standard. The only clan who is confirmed not to do so is the players, who is explicitly stated to be a very unusual clan that tardes with humans and is very open-minded.
I don't think this is supposed to be seen as an ambiguous issue. Obviously it varies from clan to clan, but this is clearly presented as something that happens fairly often.