Whilst the interaction between the Brute and Aunshi has been documented here, the remainder of the topic has a lot of additional questions asked that I feel will consistently come up again and again, and as such I thought I'd collate those thoughts into one post:
1:- Does it remove acquired keywords, such as from attachments or card effects?
Yes, since the text does not state "printed keywords"
2:- Does it remove all keywords gained after the combat phase has begun, even if it already had a copy of said keyword (such as Ranged on a Ratling Deadeye after being targetted by Preemptive Barrage)?
Yes, given the ruling on the Aun'shi thread linked above since any gain/loss of keywords recalculates the Reaction - See best answer
3:- Does it remove Area of Effect completely, or merely reduce it by one?
Given the answer to 2 is correct, it would remove Area of Effect completely - See best answer
4:- Does it affect Specialisations, such as Unstoppable?
No, since specialisations are not keywords
5:- Does it prevent Deep Strike or Ambush?
No. Cards in reserve are not units until they are deep struck, and as such are not army units to lose their keyword at the point of the reaction firing. Similar, cards in hand are not army units at said planet, and as such are also not able to lose Ambush.
6:- Does it remove Mobile?
Yes, but it doesn't stop it happening. Given that the Reaction starts just after mobile has concluded any units with mobile can move before their keyword is removed, which in effect means that removing the keyword is pointless. Currently, it would only matter should Baharroth's attachment, The Shining Blade, be attached to a mobile army unit opposite a Brute since it would cause it to be discarded.
If there's anything wrong with the answers I've given, or anything else that should be added to the list, feel free to let me know.