Maybe this is an older version habit, but I’m used to this setting - see attachment - governing my current traces width (those I’m currently laying out). No longer applicable?
By default the netclass width is used. See the section Track Width for three ways to change the width. I think you haven’t set the predefined track sizes in Board Setup so you don’t get the dropdown you expected.
You mean this?
There’s indeed three options there, but no option to switch to the .5mm I selected as my current selection.
Track width
The width of the track being routed is determined in one of three ways: if the routing start point is the end of an existing track and the 
button on the top toolbar is enabled, the width will be set to the width of the existing track. Otherwise, if the track width dropdown in the top toolbar is set to “use netclass width”, the width will be taken from the netclass of the net being routed (or from any custom design rules that specify a different width for the net, such as inside a neckdown area). Finally, if the track width dropdown is set to one of the pre-defined track sizes configured in the Board Setup dialog, this width will be used.
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That’s a grid selector, not track width selector.
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It seems as though the old cat has had a “senior” moment. 
He probably meant this:
Create all the track widths you require in:
File > Board Setup > Design Rules > Pre-defined Sizes
Then use the W or Shift + W to change widths as you draw (on the fly) or use the cursor to open the list and select a suitable width.
In my version the dropdown is labelled Grid:. Guess they decided to shorten it in v8. Nonetheless it seems OP had not predefined track widths to choose from.
Apologies @retiredfeline . It seems I need new reading glasses and my afternoon nap to give me strength to continue. 
I misinterpreted and misread the above posts. 
@Rax should end up with a list of widths if he follows my instructions
In another thread you wrote:
Learning to use netclasses properly is a skill worth learning in KiCad. It does take a bit of time to setup, but after that selecting track width is mostly automatic. A net class is a set of rules for track width and clearance and you can set up a few of them. For example start with 2. The Default Netclass which you use for almost anything, and a Power Netclass you have to add. Then you can set up this Power Netclass to use for example 1mm wide tracks to distribute some power across your PCB. In the schematic, you make a list with nets that belong to the Power Netclass, and after that, whenever you start routing on the PCB, the track width and clearance will be automatically adjusted according to the netclass of the track you are routing.
Also, netclasses just set up the defaults for the nets. If you for example need a track going to an I2C pullup resistor, then you can still use a thin track even though it is a part of the (pesumably wide) “power netclass”.