The subject says it all. I've seen people saying both, and a thread on Reddit indicating that it did, but has since been patched out. Does anyone know for sure what the current situation is: Will the two effects stack on a character using a one-handed weapon and a shield?

As long as you are wielding a single one-handed weapon (either with a shield or without one) you'll get the one-handed style crit conversion bonus.

I remember the patch and I'm very sure that now does not stack.
If you have both styles. One-handed style is only activated if you carry only one-handed single weapon without shield.

If you are wielding a shield you are no longer fighting one-handed. Why should you get a one-handed bonus? It was correctly patched out.

You're right, the patch notes state that the one-handed bonus shouldn't work with a shield. But I just finished a playthrough a few days ago (3.05) and my sabre + shield wielding fighter still triggered the one-handed bonus according to the combat log.

I would argue that fighting one-handed isn't dependent on the presence of a shield: You're using either a single-handed weapon, or a two-handed weapon. Regardless, it really should be better clarified in the description - state specifically that it's single-handed with no shield - although I suppose it's too late for that.

Anyway, to the point at hand, this is basically what I've run into elsewhere: That they're not supposed to stack, but the combat log indicates that they do. I was really hoping for something more definitive by now. :)

In the absence of proof, I think I'll just forgo it entirely. I'm not min-maxing by any measure, but there's no point wasting a skill.

Update 3.03 Patch Notes 01 June 2016

One-Handed Style: No longer works while using a shield.

https://forums.obsidian.net/blog/7/entry-193-update-notes-303/