There's no need to be confrontational.
I think that there aren't really any characters that are automatically off-limits. Most authors will probably be happy to see someone try their hand at capturing the fun parts of the NPCs that they write. The issue will probably be more about subject matter, writing quality, the nature of a portrayal, and whether the scene contradicts or preempts something else that the writer had in mind for the character. So while someone like Kasyrra isn't
technically off-limits for writing (I don't think), most scenes that new writers are likely to come up with are probably going to involve ideas that'll get rejected. When reviewing content written for NPCs by people who didn't originate those NPCs, staff tends to try to accommodate the NPCs'
creators' wishes as much as possible, and if that means saying no to a submission, they'll do that.
This isn't a hard-and-fast answer, unfortunately, but I don't think there is one. No matter what you pick, it's just good practice to ask the author. You don't
have to ask beforehand, but it's generally a good idea if you want to avoid getting your submission rejected.
The characters that are
least likely to cause issues are the ones that have relatively little content to this point, like NPCs that are solely combat encounters or side characters. Companions are also less likely to cause issues, because most of them have well-defined personalities that new authors are more likely to understand. As for "abandonware", those are relatively uncommon in the sense that almost all authors that have created NPCs for the game are also still turning writing submissions in, and even when a writer has left there may be issues with a submission for their character based on staff-determined retcons and suchlike things.
Ultimately, best practice is simply to ask about specific NPCs with concrete proposals.