Should a USB Switch work with late 2018 Macbook Air
In my investigations, I have found that for a switch to work successfully, it needs to be connected to a USB hub. Even the Apple USB/HDMI to USB-C adapter will do.
Also, plugging a USB switch after the KVM switch helps to deal with some devices (My USB mic) that seem to cause trouble when directly plugged into the KVM switch.
The two switches I have experimented with are the USB3.0 4 port 2 way KVM from UGreen and a similar one from Belkin. (No HDMI, only USB switching)
It is a little finicky, and I am not sure why. The same switch works flawlessly between by Thinkpad and Desktop (Windows)
Hope this helps.
This is what my ioreg -p IOUSB currently looks like:
+-o Root  &lt;class IORegistryEntry, id 0x100000100, retain 23&gt;
  +-o AppleUSBXHCI Root Hub Simulation@00000000  &lt;class AppleUSBRootHubDevice, id 0x100000405, registered, matched, active, busy 0 (0 ms), retain 13&gt;
  | +-o USB 10/100/1000 LAN@00200000  &lt;class AppleUSBDevice, id 0x100000407, registered, matched, active, busy 0 (0 ms), retain 14&gt;
  | +-o USB3.1 Hub@00100000  &lt;class AppleUSBDevice, id 0x1000019d1, registered, matched, active, busy 0 (0 ms), retain 12&gt;
  +-o AppleUSBXHCI Root Hub Simulation@01000000  &lt;class AppleUSBRootHubDevice, id 0x100000418, registered, matched, active, busy 0 (0 ms), retain 8&gt;
  +-o AppleUSBVHCIBCE Root Hub Simulation@80000000  &lt;class AppleUSBRootHubDevice, id 0x100000433, registered, matched, active, busy 0 (0 ms), retain 15&gt;
  | +-o FaceTime HD Camera (Built-in)@80200000  &lt;class AppleUSBDevice, id 0x100000435, registered, matched, active, busy 0 (0 ms), retain 14&gt;
  | +-o Apple T2 Controller@80100000  &lt;class AppleUSBDevice, id 0x100000439, registered, matched, active, busy 0 (0 ms), retain 13&gt;
  | +-o Apple Internal Keyboard / Trackpad@80500000  &lt;class AppleUSBDevice, id 0x100000440, registered, matched, active, busy 0 (1 ms), retain 20&gt;
  | +-o Headset@80400000  &lt;class AppleUSBDevice, id 0x100000446, registered, matched, active, busy 0 (0 ms), retain 11&gt;
  | +-o Touch Bar Display@80600000  &lt;class AppleUSBDevice, id 0x10000044a, registered, matched, active, busy 0 (0 ms), retain 13&gt;
  | +-o Ambient Light Sensor@80300000  &lt;class AppleUSBDevice, id 0x10000044e, registered, matched, active, busy 0 (0 ms), retain 11&gt;
  | +-o Touch Bar Backlight@80700000  &lt;class AppleUSBDevice, id 0x100000456, registered, matched, active, busy 0 (0 ms), retain 11&gt;
  +-o AppleUSBXHCI Root Hub Simulation@14000000  &lt;class AppleUSBRootHubDevice, id 0x10000046a, registered, matched, active, busy 0 (0 ms), retain 9&gt;
    +-o USB2.0 Hub@14100000  &lt;class AppleUSBDevice, id 0x1000019be, registered, matched, active, busy 0 (0 ms), retain 14&gt;
      +-o USB-C Digital AV Multiport Adapter@14120000  &lt;class AppleUSBDevice, id 0x1000019e2, registered, matched, active, busy 0 (3 ms), retain 12&gt;
      +-o USB2.0 Hub@14110000  &lt;class AppleUSBDevice, id 0x100002349, registered, matched, active, busy 0 (1 ms), retain 14&gt;
        +-o IOUSBHostDevice@14114000  &lt;class AppleUSBDevice, id 0x100002364, registered, matched, active, busy 0 (1 ms), retain 14&gt;
        | +-o HHKB Professional@14114100  &lt;class AppleUSBDevice, id 0x100002378, registered, matched, active, busy 0 (3 ms), retain 12&gt;
        | +-o USB Receiver@14114300  &lt;class AppleUSBDevice, id 0x10000238b, registered, matched, active, busy 0 (6 ms), retain 16&gt;
        +-o USB2.0 Hub@14111000  &lt;class AppleUSBDevice, id 0x1000023ed, registered, matched, active, busy 0 (1 ms), retain 15&gt;
          +-o Samson GoMic@14111100  &lt;class AppleUSBDevice, id 0x100002405, registered, matched, active, busy 0 (11 ms), retain 18&gt;
          +-o HD Pro Webcam C920@14111200  &lt;class AppleUSBDevice, id 0x100002454, registered, matched, active, busy 0 (13 ms), retain 19&gt;
          +-o Flash Drive@14111300  &lt;class AppleUSBDevice, id 0x100002f5a, registered, matched, active, busy 0 (4 ms), retain 12&gt;

The USB-C Digital AV Multiport (at address 0x1000019e2) was necessary for the switch to function. But it was still being finicky with my USB mic (Samson GoMic@14111100) so I added a USB2.0 Hub@14110000 (this one by Sabernet) in-line and everything is working well.