Don't think she is canonically poly, she will be what player chooses.
In my first playthrough, Tav didn't cheat on her, didn't enter relationship with Halsin ( he was always left in camp), didn't visit drow twins, and relationship was 100% monogamous.
If Tav don't cheat on her, she won't either, and she won't mention poly, i think Shadowheart (and Astarion) poly was included only because Halsin is poly (so he can be with somebody else beside Tav).
I never said she was poly. However, she shows feelings for Karlach and definitely a sexual attraction to Halsin (I've read that on act 3 she got nasty banters with Wyll but I haven't see it). But that doesn't make her poly. If you take a monogamous path with her, you can still stumble upon the twin scene and if you got Halsin on your team (and meet others conditions). Well, all I can say is 'enjoy' the view. lol.
It would be nice if at some point in the adventure you find, I don't know, a painting of an orgy and she reacts to it and let player know she approves those situations. Or better yet, let her know if you're into those things or not, and that way, avoiding they drow situation.
