So it happened.
I've had a Macbook 12" Retina 2017 laptop, the cyclops version with a single USB-C port for charge/USB/display.
About ten days or so ago, it stopped recognising USB 2.0 devices, including iPhone and old USB thumb drives. USB 3.0 devices are fine, and it still charges.
I had a look on the USB 2.0 D+ and D- lines on the USB-C connector and they appear to be diode clamped to 0.7V max, so when I apply the standard 1.5k pullup to 3.3V D+ or D- for device detection, rather than reaching the expected ~3V, they stubbornly sit at 0.7V.
So I took it to the Apple store to see how they'd deal with it. To cut a long story short, after figuring out that a $125 internal flatflex cable didn't fix it, they want $740 for a new logic board. Now I am not surprised about that, because I didn't expect the cable to be the problem bearing in mind my own diagnostics, and neither did I expect them do a component level repair.
What I did find surprising, and worrying, is that if they did do the logic board repair, I wouldn't be getting the old board back, which contains the soldered in SSD, and, of course, all my data. They do not offer a way to get the logic board back. As far as I could tell, the board goes back to Apple, where they do a component level repair, and they then re-sell it, with my data on it, presumably with a reformat and new OS install for what that's worth.
So I'm going to get it back and find a third party in the UK who'll either (a) replace the board and let me have the old one back or (b) do a component level repair. Any suggestions?