I'm Running version 1.05 on Windows 7.

If I'm understanding the description right, it says that using Expert Mode is chosen when starting a new game, and that it cannot be disabled or enabled in-game afterward.

Still, I'm able to disable Expert Mode (not expected) in any of the save games carried over from 1.04. However, after Expert Mode is disabled and the dialog window was closed, it is NOT possible to enable it again (as expected).

Has anything changed in this regard? I've not seen anything in the patch notes, or I might have missed it.

It can be disabled, it has been like this since the release version.
What you can’t do is re-enable it afterwards. (unless you start a new game of course)

So, it seems you already got how it works.

Very strange. The wiki says this: The description in game also makes me believe that you can either use or not use expert mode, setting it when the game starts, and then not be able to change the setting. In practice however, it can still be disabled, but not enabled anymore after the game has started. Having the option to disable expert mode after the game has started is not what I would expect after reading the descriptions.

The wiki is wrong on this point but the in-game description is quite clear, at least since patch 1.0.5: Maybe it was unclear in prior versions, I’d have to check.

I'm pretty sure that it was the same from the launch version. I distinctly remember the "faculties and intuition" part and that it can be disabled but not re-enabled once started.
Thankfully for that as well, because the way combat works it was a self-appointed handicap that offered no enjoyment.
Apart from the dialog checks. Damn, if you enable those the dialog window looks like shopping cart with labels in everything.

The Wiki is frequently out of date and based on pre-release information. Take it with a pinch of salt and don't trust it.

Expert Mode has always been disableable, just not re-enableable.