It looks like the Ik Fk match is missing in the viewport switch. I tested from the biped template. Matching works in synoptic.
Maya 2022.3 mgear 4.0.7 Windows 10
or am doing something silly
You need to select the host that has the switch attribute. You have one of the controls selected.
Notice “Select host” in the top of that menu. You can always click that first to quickly find/select the host, even if you don’t know where or what it is. Then after the host is selected, right click and you should see the switch.
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Thank you. Turns out I was being silly.
Not at all. I forget that about every 3 months, and start bug-hunting my rig for why the attributes aren’t showing up. 
I wonder… Maybe there is a good reason. But if the menu already “knows” what the host is for the control, maybe it could/should just show the host’s attributes when you right click on any given control.
Maybe there are some reasons that would be a bad idea. For example, if the host controls more than one component, maybe that would be less intuitive?
I’m not sure if there’s an underlinging reason or not. I’m always in favour of fewer clicks. Right clicking a control and having access to the host’s attributes would make the process more fluid and intuitive.