Has Halsin as a companion ever actually been confirmed?
Well ... actualy none of them is "confirmed" im affraid ...
Karlach was semi-confirmed by datamining, but problem with that kind of stuff is that its all subject to change ... so we can at best concider her "very much probable" but still not certain.
I never heard even single word either from Larian, or from any Dataminer about the rest that they should become actual companion. O_o
Not sure where that came from.
But there still are Helia and Minsc ...
Those should be at least simmilary probable as Karlach.
Not sure if spoiler tag is even needed, tho. :-/

I believe she's likely a paladin, so if paladin isn't ready yet, they can't have her join yet.
I would say there is most likely more "not prepared yet" about her than just that ...
(Yet another datamined content points out this way.)
Also...
If that would be really a problem for companions, then Minsc should be allready there, since Ranger was one of original implemented classes. O_o
Especialy if she is a Paladin ... it would be nothing easier to allow us to recruit her as a Cleric for now, until her class will be prepared. o_O
Same if she is suppose to be a Barbarian ... in such case they could simply give her Fighter class as a placeholder.
I still believe it would be great to release all "missing" companions that are allready planned, but dont have classes prepared yet like this ... going through the same location, doing the same quests, and picking the same (since we re-played it so many times we allready see them all) choices would be a LOT less tedious, if we could bring different companions with us, that would have different reactions on our actions. :3